WDIFF

draft-ietf-drums-smtpupd-04.txt --> draft-ietf-drums-smtpupd-05.txt

INTERNET-DRAFT John Klensin, Editor
Expires in six months MCI
March 26,
May 27, 1997

Simple Mail Transfer Protocol

draft-ietf-drums-smtpupd-04.txt

draft-ietf-drums-smtpupd-05.txt

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use
Internet-Drafts as reference material or to cite them other than as a
"working draft" or "work in progress".

To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

If consensus is reached on this document, it will be forwarded to the
IESG with the recommendation that it be processed onto the Standards
track.

[[Sections marked with doubled brackets (e.g., "<<") are explicit
placeholders or known major loose ends. The marking ## is a note in
the draft to recheck a section number and should be ignored.]]

[[As discussed in the WG, most of the syntax, and none of the
examples, have been changed from 821 -- those changes are a last-step
item, after the ABNF document completely stabilizes and differences
with 822bis have been resolved. Similarly numbers of appendices will
be rationalized (and Appendix X removed) before the document is
submitted to the IESG. Please check appendix X.2 for additional
problems of which the editor is already painfully aware.]]

TABLE OF CONTENTS
0. ABSTRACT

1. INTRODUCTION

2. THE SMTP MODEL

2.1 Basic structure
2.2 The extension model
2.3 Other terminology
2.4 Syntax Principles

3. THE SMTP PROCEDURES: AN OVERVIEW

3.1 Session initiation
3.2 Client initiation
3.3 Mail transactions
3.4 Forwarding for Address Correction or Updating
3.5 Commands for Debugging Addresses
3.6 Domains
3.7 Relaying
3.8 Mail Gatewaying
3.9 Terminating sessions and connections
3.10 Mailing lists and Aliases

4. THE SMTP SPECIFICATIONS

4.1. SMTP Commands
4.1.1. Command Semantics and Syntax
4.1.2. Lower-level Syntax
4.1.3. Address literals
4.1.4. Order of commands
4.1.5. Private-use commands
4.2. SMTP Replies
4.2.1. Reply Code Severities and Theory
4.2.2. Reply Codes by Function Group
4.2.2.
4.2.3. Reply Codes in Numeric Order
4.2.3.
4.2.4. Reply code 502
4.2.4
4.2.5 Reply codes after DATA and the subsequent CRLF.CRLF.
4.3. Sequencing of Commands and Replies
4.4 Trace information
4.5. Details
4.5.1. Minimum Implementation
4.5.2. Transparency
4.5.3. Sizes and Timeouts
4.5.4 SMTP Queuing Strategies

5. Address resolution and mail handling

6. Problem detection and handling
6.1 Reliable delivery and replies by email
6.2 Loop detection
6.3 Compensating for irregularities

7. Security Considerations
7.1 Mail security and spoofing
7.2 "Blind" copies
7.3 VRFY, EXPN, and security
7.4 Information disclosure
7.5 Scope of operation of SMTP servers

8. IANA Considerations

9. References

10. Editor's addresses

11. Acknowledgements

APPENDIX A: TCP
APPENDIX B: Generating SMTP commands from RFC 822 headers
APPENDIX C: Source routes
APPENDIX E: Theory of Reply Codes
APPENDIX F: Scenarios
APPENDIX G: Other gateway issues.
APPENDIX I: Deprecated features of RFC 821
APPENDIX X: Change summary and Loose ends (temporary)

0. Abstract

This document is a self-contained specification of the basic protocol
for the Internet electronic mail transport, consolidating and
updating

* the original SMTP specification of RFC 821 [RFC-821],
* Domain name system requirements and implications for mail
transport from RFC 1035 [RFC-DNS] and RFC 974 [RFC974],
* the clarifications and applicability statements in
RFC 1123 [RFC-1123], and
* material drawn from the SMTP Extension mechanisms [SMTPEXT].

It is intended to replace RFC 821, RFC 974, and the mail transport
materials of RFC 1123. However, RFC 821 specifies some features that
are not in significant use in the Internet of the mid-1990s and, in
appendices, some additional transport models. Those sections are
omitted in this document in the interest of clarity and brevity;
readers needing them should refer to RFC 821.

It also includes some additional material from RFC 1123 that appeared
to need amplification. These have been identified in multiple ways,
mostly by tracking flaming on the header-people list [HEADER-PEOPLE]
and problems of unusual readings or interpretations that have turned
up as the SMTP extensions have been deployed. It is important to
note that everything here is in response to some identified confusion
or bad behavior, not just paranoia.

Where this specification moves beyond consolidation and actually
differs from earlier documents, it supersedes them technically as
well as textually.

Although SMTP was designed as a mail transport and delivery protocol,
this specification also contains information that is important to its
use as a "mail posting" protocol, as recommended for POP [RFC-POP2,
RFC-POP3] and IMAP [RFC-IMAP4].

Except when the historical terminology is necessary for clarity, this
document uses the current "client" and "server" terminology to
identify the sending and receiving SMTP processes, respectively.

A companion document discusses message bodies and formats RFC 822,
MIME, and their relationship - [MSGFMT].

1. INTRODUCTION

The objective of the Simple Mail Transfer Protocol (SMTP) is to
transfer mail reliably and efficiently.

SMTP is independent of the particular transmission subsystem and
requires only a reliable ordered data stream channel. While this
document specifically discusses transport over TCP, other transports
are possible. Appendices to RFC 821 describe some of them. Section
##2.3 provides definitions of terms specific to this document.

An important feature of SMTP is its capability to transport mail
across transport service environments, usually referred to as "mail
gatewaying".
gatewaying" (see section 3.8). A transport service environment might
consist of the mutually-TCP-accessible hosts on the public internet,
a firewall-isolated private TCP/IP LAN, or a LAN or WAN environment
utilizing an entirely different transport-level protocol. It is
important to realize that transport systems are not one-to-one with
usual definitions of "networks". A process can communicate directly
with another process, and transport mail using this protocol, through
any mutually known transport layer. Conversely, mail can be relayed
(actually gatewayed) between hosts on different transport systems by
a host on both transport systems. The Mail eXchanger mechanisms of
the domain name system [RFC-DNS, and section 5 of this document]
usually permit relaying and gatewaying to occur invisibly to the user.

2. THE SMTP MODEL

2.1 Basic structure

The SMTP design is based on the following model of communication: as
the result of a user mail request (or transfer from a mail user agent
(see section ##2.3)), the SMTP client establishes a two-way
transmission channel to an SMTP server. Fully-capable client SMTPs
determine the host address supporting the server SMTP function by
resolving the domain name in the request to it into either an
intermediate mail exchanger host or a final target host. In other
cases, common with clients associated with implementations of the POP
[RFC-POP2, RFC-POP3] or IMAP [RFC-IMAP4] protocols, or when the
client is inside an isolated transport service enviroment, the SMTP
client may send all of its traffic to a single SMTP server which, in
turn, relays the mail to final (or other intermediate) destinations.
The relay and those destinations in turn are expected to support all
of the queuing, retrying, and alternate address functions discussed
in this specification. The SMTP server may be either the ultimate
destination or an intermediate "relay" (i.e., may assume the role of
an SMTP client after receiving the message). SMTP commands are
generated by the SMTP client and sent to the SMTP server. SMTP
replies are sent from the SMTP server to the SMTP client in response
to the commands.

Once the transmission channel is established and initial handshaking
completed, the SMTP-client normally initiates a mail transaction.
Such a transaction consists of a series of commands to specify the
originator and destination of the mail and transmission of the
message content (including any headers or other structure) itself.
When the same message is sent to multiple recipients this protocol
encourages the transmission of only one copy of the data for all the
recipients at the same destination (or intermediate relay) host.

The server responds to each command with a reply; replies may
indicate that the command was accepted, that additional commands are
expected, or that a temporary or permanent error condition exists.
Commands that specify the sender or recipients may include
server-permitted SMTP service extension requests as discussed in
section ##2.2. The dialog is purposely lock-step, one-at-a-time
although this can be modified by mutually-agreed extension requests
(e.g., [RFC-Pipeline]).

Once a given mail message has been transmitted, the client may either
request that the connection be shut down or may initiate other mail
transactions.

-------------------------------------------------------------

+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+

SMTP client SMTP server

Model for SMTP Use

Figure 1

-------------------------------------------------------------

Less commonly, the SMTP protocol and connection may be used by the
client to request ancillary services of the server such as
verification of addresses or exhibiting the contents of mailing lists.

As suggested above, this protocol provides mechanisms for the
transmission of mail. This transmission normally occurs directly
from the sending user's host to the receiving user's host when the
two hosts are connected to the same transport service. When they are
not connected to the same transport service, transmission occurs via
one or more relay SMTP-servers. An intermediate host that will act
as either an SMTP relay or as a gateway into some other transmission
environment may also be selected through the use of the domain name
service (DNS) Mail eXchanger mechanism.

To be able to provide the relay capability the server SMTP is
supplied with the name of the ultimate destination host as well as
the destination mailbox name. Usually, intermediate hosts are
determined via the DNS MX record, not by explicit "source" routing
(see Appendices ##C and ##I).

2.2 The Extension Model

2.2.1 Background

In an effort that started in 1990, approximately a decade after RFC
821 was completed, the protocol was modified with a "service
extensions" model that permits the client and server to agree to
utilize shared functionality that goes beyond the original basic SMTP
requirements. Contemporary SMTP implementations MUST support the
basic extension mechanisms (see below for details), i.e., servers
MUST support the EHLO command even if they do not implement any
specific extensions and clients MUST preferentially utilize EHLO
rather than HELO. However, for compatibility with older
implementations (which are expected to persist for some years), SMTP
clients and servers MUST support the original HELO mechanisms as a
fallback.

Although SMTP is widely and robustly deployed, some parts of the
Internet community might wish to extend the SMTP service. The SMTP
extension mechanism defines a means whereby an extended SMTP client
and server may recognize each other as such and the server can inform
the client as to the service extensions that it supports.

It must be emphasized that any extension to the SMTP service should
not be considered lightly. SMTP's strength comes primarily from its
simplicity. Experience with many protocols has shown that:

protocols with few options tend towards ubiquity, whilst
protocols with many options tend towards obscurity.

This means that each and every extension, regardless of its benefits,
must be carefully scrutinized with respect to its implementation,
deployment, and interoperability costs. In many cases, the cost of
extending the SMTP service will likely outweigh the benefit.

Given this environment, the extension framework consists of:

(1) The SMTP command EHLO, superseding the earlier HELO,

(2) a registry of SMTP service extensions, and

(3) additional parameters to the SMTP MAIL FROM and RCPT TO
commands.

(4) optional replacements for verbs defined in this protocol,
such as for DATA (e.g., see [RFC-BDAT]).

2.2.2 Definition and Registration of Extensions

The IANA maintains a registry of SMTP service extensions. Associated
with each such extension is a corresponding EHLO keyword value. Each
service extension registered with the IANA must be defined in an RFC.
Such RFCs must either be on the standards-track or must define an
IESG-approved experimental protocol. The definition must include:

(1) the textual name of the SMTP service extension;

(2) the EHLO keyword value associated with the extension;

(3) the syntax and possible values of parameters associated
with the EHLO keyword value;

(4) any additional SMTP verbs associated with the extension
(additional verbs will usually be, but are not required
to be, the same as the EHLO keyword value);

(5) any new parameters the extension associates with the
MAIL FROM or RCPT TO verbs;

(6) how support for the extension affects the behavior of a
server and client SMTP; and,

(7) the increment by which the extension is increasing the
maximum length of the commands MAIL FROM, RCPT TO, or
both, over that specified in RFC 821.

In addition, any EHLO keyword value that starts with an upper or
lower case "X" refers to a local SMTP service extension, which is
used through bilateral agreement, rather than being standardized.
Keywords beginning with "X" may not be used in a registered service
extension.

Any Conversely, keyword values presented in the EHLO response
that do not begin with "X" must correspond to a standard,
standards-track, or IESG-approved experimental SMTP service extension
registered with IANA. A conforming server must not offer
non-"X"-prefixed keyword values that are not described in a
registered extension.

Additional verbs and parameter names are bound by the same rules as
EHLO keywords; specifically, verbs begining with "X" are local
extensions that may not be registered or standardized. Conversely,
verbs not beginning with "X" must always be registered.

2.3 Terminology

Most of the terminology in this document is common in the Internet at
the time of its writing. However, the following terms and concepts
are used in special ways here, or represent differences in
terminology between RFC 821 and this document, and should be
understood before reading further. These definitions are normative,
i.e., they contain specifications to which SMTP implementations are
required to conform.

2.3.1 Mail objects

SMTP relays a mail object containing an envelope and a content.

(1) The SMTP envelope is straightforward, and is sent as a
series of SMTP protocol units (described in section ##3): it
consists of an originator address (to which error reports
should be directed); a delivery mode (e.g., deliver to
recipient mailboxes); one or more recipient addresses; and
optional protocol extension material.

(2) The SMTP content is sent in the SMTP DATA protocol unit
and has two parts: the headers and the body. The
headers form a collection of field/value pairs
structured as described in [MSGFMT], whilst the body,
if structured, is defined according to MIME [3]. The
content is textual in nature, expressed using the US
ASCII repertoire (ANSI X3.4-1986). Although extensions
(such as MIME) may relax this restriction for the
content body, the content headers are always encoded
using the US ASCII repertoire. The algorithm defined in
[4] is used to represent header values outside the US
ASCII repertoire, whilst still encoding them using the
US ASCII repertoire.

2.3.2. Sender and receivers

In RFC 821, the two hosts participating in an SMTP transaction were
described as the "SMTP-sender" and "SMTP-receiver". This document
has been changed to reflect current industry terminology and hence
refers to them as the "SMTP client" (or sometimes just "the client")
and "SMTP server" (or just "the server") respectively. Since a given
host may act both as server and client in a relay situation,
"receiver" and "sender" terminology is still used where needed for
clarity.

2.3.3. Mail agents

Additional mail system terminology became common after RFC 821 was
published and, where convenient, is used in this specification. In
particular, SMTP servers and clients provide a mail transport service
and therefore act as Mail Transfer Agents (MTAs). Mail User Agents
(MUAs or UAs) are normally thought of as the sources and targets of
mail. At the source, an MUA might collect mail to be transmitted
from a user and hand it off to an MTA; the final ("delivery") MTA
would be thought of as handing the mail off to an MUA (or at least
transferring responsibility to it). However, while these terms are
used with at least the appearance of great precision in other
environments, the implied boundaries between MUAs and MTAs often do
not accurately match common, and conforming, practices with Internet
mail. Hence, the reader should be cautious about inferring the
strong relationships and responsibilities that might be implied if
these terms were used elsewhere.

2.3.4 host

For the purposes of this specification, a host is a computer system
attached to the Internet (or, in some cases, to a private TCP/IP
network) and supporting the SMTP protocol. Hosts are known by names
(see "domain"); identifying them by address is discouraged.

2.3.5 domain

The name of a host (often referred to as a "fully-qualified domain
name" or "FQDN"), or some entry in the domain name hierarchy, usually
referred to as a "subdomain", that may contain many hosts. A domain,
or domain name, may also refer to an alias ("CNAME") or may provide a
reference to Mail eXchanger records to be used to deliver mail. See
[RFC-DNS] and section ##5.

Some published material about the Internet (and about Internet mail
in particular) refers to a "host in a domain" as if the host name
were an unqualified name with a domain name appended to it. This
convention has caused considerable confusion and some protocol
violations: the domain name, as described in this document and in
[RFC-DNS], is the entire, fully-qualified name, and an apparent host
name that is not in FQDN form is no more than a local alias. Local
aliases MUST NOT appear in any SMTP transaction.

2.3.6 buffer and state table

The SMTP protocol is described in terms of an can be modelled by a simple abstract machine that
accumulates some information as the various in
which commands are issued as
well as remembering, exchanged between client and server. Some
commands cause the server to accumulate information in a virtual
buffer, other commands cause that buffer to be cleared, or the extent necessary, data
in the sequence buffer to be acted upon, causing mail to be transmitted.
Because of this relationship, it is important that commands be
correctly sequenced, and to ensure this the commands anticipated next. Using server will also have a
notion of the terminology current state of
one possible implementation model, communications. Commands alter that
state in a well-defined sequence, or reset the storage for information
accumulation is referred sequence to in an earlier
point. In this document as the document, "buffer" refers to this virtual buffer and
"state table" to the
history list of commands and anticipated actions is referred which caused the server to
achieve its current state. Operations described as the
"state table". The operations of "clearing the
buffer" or "resetting the state table" involve restoring that accumulated cause the information in the
buffer to
their status be discarded and content before the relevant commands were issued. state to be returned to that which
existed at an earlier time.

2.3.7 lines

SMTP commands and, unless altered by a service extension, message
data, are transmitted in "lines". Lines are defined as consisting
zero or more data characters terminated by the ASCII sequence of "CR"
followed immediately by "LF". Conforming implementations MUST NOT
recognize any other character or character sequence as a line
terminator.

2.3.8 Gateway, relay, originator, and delivery system

This specification makes a distinction among four types of SMTP
systems, based on the role those systems play in transmitting
electronic mail. An "originating" system (sometimes called an
SMTP-originator) is one that introduces mail into the Internet or,
more generally, into a transport service environment. When the
concept of a mail user agent is useful, the SMTP-originator is the
mail transfer agent to which an originating mail user agent hands off
the message. A "delivery" SMTP system is one that receives mail from
a transport service environment and hands it to a mail user agent or
deposits it in a maildrop which a mail user agent is expected to
subsequently access. A "relay" SMTP system (usually referred to just
as a "relay") receives mail from an SMTP client and transmits it,
without modification to the message data other than adding trace
information, to another SMTP server for further relaying or for
delivery.

A "gateway" SMTP system (usually referred to just as a "gateway")
receives mail from a client system in one transport environment and
transmits it to a server system in another transport environment.
sending. While either the client or server system must use the SMTP
service in order for this document to be relevant, there is no
requirement in a gateway environment that both do so and, typically,
one will not. Differences in protocols or message semantics between
the transport environments on either side of a gateway may require
that the gateway system perform transformations to the message that
are not permitted to SMTP relay systems.

2.3.9 Message content and message body

The terms "message content" and "mail data" are used interchangably
in this document to describe the material transmitted after the DATA
command is accepted and before the end of data indication is
transmitted. Message content includes message headers and the
possibly-structured message body. See ##2.3.1.

2.3.10 mailbox

A and address

As used in this specification, an "address" is a character string (address)
which identifies a user or other
location to whom mail is to be sent. A mailbox sent or a location into
which mail is to be deposited. The term "mailbox" refers to that
depository. The two terms are typically used interchangably unless
the distinction between the location in which mail is placed (the
mailbox) and a reference to it (the address) is important. An
address normally consists of user and domain specifications. The
standard mailbox naming convention is defined to be
"local-part@domain": contemporary usage permits a much broader set of
applications than simple "user names" and, consequently, the
local-part is permitted to be interpreted and assigned semantics only
by the host specified in the domain part of the address.
Additionally, the "container" in which mail is stored. <<>>Is this
terminology now consistent with the Message Format document??>>

2.3.11. reply

A reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a
command. The general form of a reply is a completion code (including
error codes) followed by a text string. The codes are for use by
programs and the text is usually intended for human users.

2.4 Syntax Principles

2.4.1 General syntax and transaction model

The mail commands and replies have a rigid syntax. Replies also have
a numeric code. In the following, examples appear which use actual
commands and replies. The complete lists of commands and replies
appears
appear in Section ##4 on specifications. "The SMTP Specification".

Commands and replies are not case sensitive. That is, a command or
reply word MAY be upper case, lower case, or any mixture of upper and
lower case. Note that this is not true of mailbox user names. For
some hosts the user name is case sensitive (this practice impedes
interoperability and is discouraged), and SMTP implementations MUST
take care to preserve the case of user names as they appear in
mailbox arguments. Domain names are not case sensitive.

Commands and replies are composed of characters from the ASCII
character set [1]. When the transport service provides an 8-bit byte
(octet) transmission channel, each 7-bit character is transmitted
right justified in an octet with the high order bit cleared to zero.
More specifically, the unextended SMTP service provides seven bit
transport only. Originating SMTP clients MUST NOT transmit messages
with information in the high-order bit of octets. If such messages
are transmitted in violation of this rule, receiving SMTP servers MAY
clear the high-order bit or reject the message as invalid. In
general, a relay SMTP SHOULD assume that the message content it has
received is valid and, assuming that the envelope permits doing so
relay it without inspecting that content. Of course, if the content
is mislabelled and the data path cannot accept the actual content,
this may result in ultimate delivery of a severely garbled message by
the recipient and delivery SMTP systems MAY reject ("bounce") such
messages rather than delivering them. No sending SMTP system is
permitted to send envelope commands in any character set other than
US-ASCII; receiving systems SHOULD reject such commands, normally
using "500 syntax error - invalid character" replies.

Eight-bit message content transmission MAY be requested of the server
by the client using extended SMTP facilities, notably the "8BITMIME"
extension [8BITMIME]. 8BITMIME SHOULD be supported by SMTP servers.
However, it MUST not be construed as authorization to transmit
unrestricted eight bit material; for material with the high bit on
that is not in MIME format with an appropriate content-transfer
encoding, "8BITMIME" MUST NOT be requested by senders and servers MAY
respond as discussed above.

The metalinguistic notation used in this document corresponds to the
"Augmented BNF" used in other Internet mail system documents. The
reader who is not familiar with that syntax should consult [ABNF].

2.4.2 Command and reply syntax

The commands consist of a command code followed by an argument field.
Command codes are four alphabetic characters. Upper and lower case
alphabetic characters are to be treated identically. Thus, any of
the following may represent the mail command:

MAIL Mail mail MaIl mAIl

This also applies to any symbols representing parameter values, such
as "TO" or "to" for the forward-path. Command codes and the argument
fields are separated by one or more spaces. However, in the
local-part within the reverse-path and forward-path arguments case is
important. In particular, in some hosts the user "smith" is
different from the user "Smith".

A few SMTP receiver systems, in violation of this specification (and
RFC 821) require that a particular case be transmitted by clients.
Implementations MAY wish to make provision to accomodate those
systems.

The argument field consists of a variable length character string
ending with the character sequence <CRLF>. The receiver is to take
no action until this sequence is received.

The syntax for each command is shown with the discussion of that
command. Common elements and parameters are shown in section ##4.1.2.

3. THE SMTP PROCEDURES: AN OVERVIEW

This section presents the procedures used in SMTP in several parts.
After a review of session initiation by the server and client, there
is the basic mail procedure defined as a mail transaction. Following
this are descriptions of forwarding mail, verifying mailbox names and
expanding mailing lists, and the opening and closing exchanges. At
the end of this section are comments on relaying, a note on mail
domains, and a discussion of changing roles. Throughout this section
are examples of partial command and reply sequences; several complete
scenarios are presented in Appendix ##F.

3.1 Session initiation

An SMTP session is initiated by the client opening a connection to
the server and the server responding with an opening message.

SMTP server implementations MAY include identification of their
software and version information in the connection greeting reply
after the 220 code (see section ##7.4. This practice permits much
more efficient isolation and repair of any problems. While some
systems also identify their contact point for mail problems, this is
not a substitute for maintaining the required Postmaster address (see
[RFC822]). Implementations MAY make provision for SMTP servers to be
configured to disable the software and version announcement where it
causes security concerns.

3.2 Client initiation: EHLO

The client

In some cases, a server host may wish to explicitly indicate that
SMTP is not supported, or is not supported for connections
originating from the would-be sender's address, rather than merely
not having a listener on the relevant port and letting the client
timeout (and potentially retry repeatedly). In that case, a 554
response MAY be given in the initial connection opening message
instead of the 220. A server taking this approach MUST still wait
for the client to send a QUIT (see section ##4.1.1.10) before closing
the connection and SHOULD respond to any intervening commands with
"503 bad sequence of commands". Since an attempt to make an SMTP
connection to such a system is probably in error, a server returing a
554 response on connection opening SHOULD provide enough information
in the reply text to facilitate debugging of the sending system.

<<>> Note in draft: I'd really like to invent code 521 instead of
overloading 554 for this case -- opinions?? <<>>

3.2 Client initiation: EHLO

The client then sends the EHLO command to the server, indicating its
identity. In addition to opening the session, use of EHLO indicates
that the client is able to process service extensions and requests
that the server provide a list of the extensions it supports. Older
SMTP systems, unable to support service extensions, MAY use HELO
instead of EHLO but EHLO SHOULD be used by all current clients and
accepted by all current systems.

In the EHLO, or the older HELO, command the host sending the command
identifies itself; the command may be interpreted as saying "Hello, I
am <domain>" (and, in the case of EHLO, "and I support service
extension requests").

3.3. Mail Transactions

There are three steps to SMTP mail transactions. The transaction is
started with a MAIL command which gives the sender identification. A
series of one or more RCPT commands follows giving the receiver
information. Then a DATA command gives the mail data. And finally,
the end of mail data indicator confirms the transaction.

The first step in the procedure is the MAIL command. The
<reverse-path> contains the source mailbox.

MAIL <SP> FROM:<reverse-path> [<SP> <mail-parameters>] <CRLF>

This command tells the SMTP-receiver that a new mail
transaction is starting and to reset all its state tables and
buffers, including any recipients or mail data. It gives the
reverse-path which can be used to report errors (see section
##4.2 for a discussion of error reporting). If accepted, the
SMTP server returns a 250 OK reply. If the mailbox specification
is not acceptable for some reason, the server MAY return a 550 or
553 reply.

While, historically, the <reverse-path> can contain more than just
a mailbox, contemporary systems SHOULD NOT use source routing (see
Appendix ##C).

The optional <mail-parameters> are associated with negotiated SMTP
service extensions (see section ##2.2).

The second step in the procedure is the RCPT command.

RCPT <SP> TO:<forward-path> [<SP> <rcpt-parameters>] <CRLF>

This command gives a forward-path (normally a mailbox and domain)
identifying one recipient. If accepted, the SMTP server returns a
250 OK reply, and stores the forward-path. If the recipient is
unknown the SMTP server returns a 550 Failure reply (other
circumstances and reply codes are possible). This second step of
the procedure can be repeated any number of times. The
<forward-path> can contain more than just a mailbox. While,
historically, the <forward-path> may be a source routing list of
hosts and the destination mailbox, contemporary SMTP clients
SHOULD NOT utilize source routes (see Appendix ##C). Servers MUST
be prepared to encounter a list of source routes in the forward
path, but SHOULD ignore the routes or MAY decline to support the
relaying they imply. Similarly, servers MAY decline to accept
mail that is destined for other hosts or systems. Of course, such
a restrictions would make a server useless as a relay for clients
that do not support full SMTP functionality, but such clients MUST
NOT assume that any SMTP server on the Internet can be used as
their mail processing site. If RCPT TO appears without a
previous MAIL FROM, the server MUST return a 503 "Bad sequence of
commands" response.

Clients SHOULD NOT utilize explicit source routing except as
discussed in Appendix ##C.

The optional <mail-parameters> are associated with negotiated SMTP
service extensions (see section ##2.2).

The third step in the procedure is the DATA command (or some
alternative specified in a service extension).

DATA <CRLF>

If accepted, the SMTP server returns a 354 Intermediate reply
and considers all succeeding lines to be the message text.
When the end of text is received and stored the SMTP-receiver
sends a 250 OK reply.

Since the mail data is sent on the transmission channel, the
end of the mail data must be indicated so that the command and
reply dialog can be resumed. SMTP indicates the end of the mail
data by sending a line containing only "." (period or full
stop). A transparency procedure is used to prevent this from
interfering with the user's text (see Section ##4.5.2).

The end of mail data indicator also confirms the mail transaction
and tells the SMTP server to now process the stored recipients and
mail data. If accepted, the SMTP server returns a 250 OK reply.
The DATA command should can fail in only if the mail transaction two ways:

o If there was
incomplete (for example, no recipients), MAIL FROM, or if resources are not
available. However, some servers in practice do not perform
recipient verification until after the message text is received.
These servers SHOULD treat a failure for one no RCPT TO, commands, or more recipients as
a "subsequent failure" and all
such commands were rejected, the server MAY return a mail message as "command
out of sequence" (503) reply. If that reply is received,
the client MUST NOT send the message data; more generally,
message data MUST NOT be sent unless a 354 reply is received.

o If the verb is initially accepted and the 354 reply issued,
the DATA command should fail only if the mail transaction was
incomplete (for example, no recipients), or if resources are
not available. However, some servers in practice do not
perform recipient verification until after the message text
is received. These servers SHOULD treat a failure for one or
more recipients as a "subsequent failure" and return a mail
message as discussed in section ##6. Using a "550 mailbox
not found" (or equivalent) reply code after the data are
accepted makes it difficult or impossible for the client to
determine which recipients failed.

Please note that, when

When RFC 822 format is being used, the mail data includes the memo
header items such as Date, Subject, To, Cc, From [RFC822]. Server
SMTP systems SHOULD NOT reject messages based on perceived defects in
the RFC 822 or MIME [MIME] message header or message body. In
particular, they MUST NOT reject messages on the basis of trying to
match numbers of Resent- fields. In particular, messages MUST NOT be
rejected because Resent-to appears without Resent-from, Resent-date,
or both.

The above procedure is an example of a mail transaction. These
commands must be used only in the order discussed above. Example 1
(below) illustrates the use of these commands in a mail transaction.

3.4. Forwarding for Address Correction or Updating

The "forwarding" mechanisms described in section 3.2 of RFC 821, and
especially the 251 reply code from RCPT TO that indicates a corrected
destination, are no longer in active use. Forwarding support is most
often required to consolidate and simplify addresses within, or
relative to, some enterprise. In most of those cases, information
hiding (and sometimes security) considerations argue against exposure
of the "final" address through the SMTP protocol as a consequence of
the forwarding activity and, in some cases, that final address may
not even be reachable by the sender.

Silent forwarding of messages (without server notification to the
sender) is common in the contemporary Internet.

If the forwarding and address correction mechanisms described in RFC
821 are used, the addresses given should be stable enough that it
would be reasonable for the client to update local records with them.

3.5. Commands for Debugging Addresses

3.5.1 Overview

SMTP provides, as additional features, commands to verify a user name
or expand a mailing list. This is done with the VRFY and EXPN
commands, which have character string arguments. Implementations
MUST support VRFY and SHOULD support EXPN (however, see section
##3.5.2 and ##7.3). For the VRFY command, the string is a user name
or a user name and domain (see below) and the response may include
the full name of the user and must include the mailbox of the user,
e.g., it MUST BE in either
User Name <mailbox@domain>
or
mailbox@domain form.

Paths (explicit source routes) MUST NOT be returned by VRFY or EXPN.

When a name that is the argument to VRFY could identify more than one
mailbox, the server MAY either note the ambiguity or identify the
alternatives. In other words, either of the following are legitimate
response to VRFY:

553 User ambiguous
or
553- Ambiguous; Possibilities are
553-Joe Smith <jsmith@somedomain>
553-Harry Smith <hsmith@somedomain>
553 Melvin Smith <dweep@somedomain>

Under normal circumstances a client receiving a 553 reply would be
expected to expose the result to the user. Use of exactly the forms
given, and the "user ambiguous" or "ambiguous" keywords, possibly
supplemented by extended reply codes as described in [RFC-REPLY],
will facilitate automated translation into other languages as needed.

For the EXPN command, the string identifies a mailing list, and the
multiline response MAY include the full name of the users and MUST
give the mailboxes on the mailing list.

"User name" is a fuzzy term and used purposely. An implementation of
the VRFY or EXPN commands MUST include at least recognition of local
mailboxes as "user names". A However, since current Internet practice
often results in a single host handling mail for multiple domains,
hosts, especially hosts that provide this functionality, SHOULD
accept the "user@domain" form as a "user name"; hosts MAY also choose
to recognize other strings as "user names".

In some hosts the distinction between a mailing list and an alias for
a single mailbox is a bit fuzzy, since a common data structure may
hold both types of entries, and it is possible to have mailing lists
of one mailbox. If a request is made to verify a mailing list a
positive response can be given if on receipt of a message so
addressed it will be delivered to everyone on the list, otherwise an
error should be reported (e.g., "550 That is a mailing list, not a
user"). If a request is made to expand a user name, the server MAY
return a positive response consisting of a list containing one name,
or an error MAY be reported (e.g., "550 That is a user name, not a
mailing list").

In the case of a multiline reply (normal for EXPN) exactly one
mailbox is to be specified on each line of the reply. The case of an
ambiguous request is discussed above.

The case of verifying a user name is straightforward as shown in
example 3.

The case of expanding a mailbox list requires a multiline reply as
shown in example 4.

The character string arguments of the VRFY and EXPN commands
cannot be further restricted due to the variety of implementations
of the user name and mailbox list concepts. On some systems it
may be appropriate for the argument of the EXPN command to be a
file name for a file containing a mailing list, but again there is
a variety of file naming conventions in the Internet.

3.5.2 VRFY normal response.

When normal (2yz or 551) responses are returned from a VRFY or EXPN
request, the reply should normally include the mailbox name, e.g.,
"<foo@bar>" (where "bar" is a fully qualified domain name) must
appear in the syntax. In exceptional circumstances, free-form text
MAY be returned. In order to facilitate parsing by both computers
and people, addresses SHOULD appear in pointed brackets. EXPN and
VRFY MUST return only valid domain addresses that are usable in SMTP
RCPT commands. Consequently, if an address implies delivery to a
program or other system, the mailbox name used to reach that target
MUST be given.

Server implementations MUST support VRFY and SHOULD support EXPN.
For security reasons, implementations MAY provide local installations
a way to disable either or both of these commands through
configuration options or the equivalent. When these commands are
supported, they are not required to work across relays when relaying
is supported. Since they were both optional in RFC 821, they MUST,
if supported, be listed in the response to EHLO if service extensions
are supported.

3.5.3 Meaning of VRFY or EXPN success response.

A server MUST NOT return a 220 code in response to a VRFY or EXPN
command unless it has actually verified the address. In particular,
a server MUST NOT return 220 if all it has done is to verify that the
syntax given is valid. In that case 502 (Command not implemented) or
500 (Syntax error, command unrecognized) SHOULD be returned (note
that returned. As
provided elsewhere, implementation of VRFY is required by RFC 1123 and EXPN is
strongly recommended; this specification does not change that
requirement and, hence, recommended. Hence, except as provided in section ##7.3,
implementations that return 500 or 502 for VRFY are not in compliance
with these specifications). this specification.

Especially when a server is acting as a mail exchanger for another,
there may be circumstances where an address appears to be correct valid but
cannot reasonably be verified in real time. In "Apparent validity" in
this case would normally involve at least syntax checking and might
involve verification that situation, any domains specified were one for which
the host expected to be able to relay mail. In these situations,
reply code 252 SHOULD BE returned. These cases parallel the
discussion of RCPT verification discussed in section ##2.1 although
implementations generally SHOULD be more aggressive about address
verification in the case of VRFY than in the case of RCPT even if a
little more time is required to do so.

3.5.4. Semantics and applications of EXPN.

While EXPN is often very useful in debugging and understanding
problems with mailing lists and multiple-target-address aliases, some
systems have attempted to use source expansion of mailing lists as a
means of eliminating duplicates. The propagation of aliasing systems
with mail on the Internet--both for hosts (typically with MX and
CNAME DNS records) and for mailboxes (various types of local host
aliases) has made it nearly impossible for these strategies to work,
and mail systems SHOULD NOT attempt them.

3.6. Domains

Domains have become a key concept in the Internet mail system. The
use of domains changes the address space from a flat global space of
simple character string host names to a hierarchically structured
rooted tree of global addresses. The host name is replaced by a
domain designator which is a sequence of domain element strings
separated by periods with the understanding that the domain elements
are ordered from the most specific to the most general.

For example, "ISIF.ISI.EDU", "Fred.Cambridge.AC.UK", and
"PC7.LCS.MIT.EDU" might be domain identifiers.

Whenever domain names are used in SMTP, only resolvable,
fully-qualified, domain names (FQDNs) are permitted. In other words,
names that can be resolved to MX RRs or A RRs (as discussed in
section ##5.??.??) ##5) are permitted, as are CNAME RRs whose targets can be
resolved, in turn, to MX or A RRs. Local nicknames or unqualified
names MUST NOT be used. There is one exception are two exceptions to this rule: the (i)
The domain name given in the EHLO (or HELO) command MUST BE either a
primary host name (a domain name that resolves to an A RR) or, if the
host has no name, a domain an address literal as described in section ##___.
##4.1.1.1 and (ii) The reserved mailbox name "Postmaster" may be used
in a RCPT TO command without domain qualification (see section
##4.1.1.3).

3.7. RELAYING

In general, the availability of Mail eXchanger records in the domain
name system [RFC-DNS] makes the use of explicit source routes in the
Internet mail system unnecessary. Many historical problems with
their interpretation have made their use undesirable. SMTP clients
SHOULD NOT generate explicit source routes except under unusual
circumstances. SMTP servers MAY decline to act as mail relays or to
accept addresses that specify source routes. They are also permitted
to ignore the route information and simply send to the final
destination as specified in the route and the DNS. However, there
has been a practice, albeit invalid, of using names that do not
appear in the DNS as destination names, with the senders counting on
the intermediate hosts specified in source routing to resolve any
problems. If source routes are stripped, this practice will cause
failures -- one of several reasons why SMTP clients MUST NOT generate
invalid source routes or depend on serial resolution of names.

If source routes are not used, the process described in RFC 821 for
constructing a reverse-path from the forward-path is not applicable
and the reverse-path at the time of delivery will simply be the
address that appeared in the MAIL command.

A relay SMTP server is usually the target of a DNS MX record that
designates it rather than the final delivery system. It receives
mail to be relayed to another SMTP server. The relay server may
accept or reject the task of relaying the mail in the same way it
accepts or rejects mail for a local user. If it accepts the task, it
then becomes an SMTP client, establishes a transmission channel to
the next SMTP server specified in the DNS (according to the rules in
section ##___), ##5), and sends it the mail.

If an SMTP server has accepted the task of relaying the mail and
later finds that the destination is incorrect or that the mail cannot
be delivered for some other reason, then it MUST construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the
reverse-path). Formats specified for non-delivery reports by other
standards SHOULD be used if possible.

This notification message must be from the SMTP server at the relay
host or the host that first determines that delivery cannot be
accomplished. Of course, SMTP servers MUST NOT send notification
messages about problems with notification messages. One way to
prevent loops in error reporting is to specify a null reverse-path in
the MAIL command of a notification message. When such a message is
transmitted the reverse-path MUST BE set to null. A MAIL command
with a null reverse-path appears as follows:

MAIL FROM:<>

An undeliverable mail notification message is shown in example 7.
This notification is in response to a message originated by JOE at
HOSTW and sent via HOSTX to HOSTY with instructions to relay it on to
HOSTZ. What we see in the example is the transaction between HOSTY
and HOSTX, which is the first step in the return of the notification
message.

3.8. Terminating Sessions and Connections

An SMTP connection is terminated by

3.8 Mail Gatewaying

While the client's sending a QUIT
command. The server then responds with a positive reply code, after
which it closes relay function discussed above hands mail off within the connection.

An
Internet SMTP transport service environment, either MX records or
various forms of explicit routing may require that an intermediate
SMTP server MUST NOT intentionally close the connection
except:
o After receiving perform a QUIT command translation function between one transport
service and responding with another. As discussed in section ##2.3.8, when such a 221
reply.
o After detecting the need to shutdown
system is at the SMTP boundary between two transport service
and returning a 451 reply environments,
we refer to any command.

In particular, it as a server that closes connections in response
to commands that are not understood "gateway" or "gateway SMTP".

Gatewaying mail between different mail environments, i.e., different
mail formats and protocols, is in violation of this
specification. Instead, servers are expected complex and does not easily yield to
standardization. However, some general requirements may be tolerant of
unknown commands, issuing given for
a 500 reply gateway between the Internet and awaiting further
instructions from another mail environment.

3.8.1 Header fields MAY be rewritten when necessary as are gatewayed
across mail environment boundaries.

This may involve inspecting the client.

An SMTP server which is forcibly shut down via external
means SHOULD attempt message body or interpreting
the local-part of the destination address in spite of the
prohibitions in ##___.

Other mail systems gatewayed to send the Internet often use
a line containing 451 response
code subset of RFC-822 headers or their functionality, but some of
them do not have an equivalent to the SMTP client before exiting. The envelope.
Therefore, when a message leaves the Internet environment, it
may be necessary to fold the SMTP client will
normally read envelope information into the 451 response code after sending its next
command.

4. THE SMTP SPECIFICATIONS

4.1. SMTP COMMANDS

4.1.1. COMMAND SEMANTICS AND SYNTAX

The SMTP commands define
message header. A possible solution would be to create new
header fields to carry the envelope information (e.g.,
"X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would require
changes in mail transfer programs in the foreign environment.

3.8.2 When forwarding a message into or out of the mail system
function requested by Internet
environment, a gateway MUST prepend a Received: line, but it
MUST NOT alter in any way a Received: line that is already in
the user. SMTP commands are character
strings terminated by <CRLF>. The command codes themselves are
alphabetic characters terminated by <SP> if parameters follow
and <CRLF> otherwise. The syntax header.

"Received:" fields of mailboxes (local parts)
must messages originating from other
environments may not conform exactly to receiver site conventions and this specification.
However, the syntax
specified in section ##__. The SMTP commands are discussed
below. The SMTP replies are discussed in Section ##4.2.

A most important use of Received: lines is for
debugging mail transaction involves several data objects which are
communicated as arguments faults, and this debugging can be severely
hampered by well-meaning gateways that try to different commands. The
reverse-path is the argument "fix" a Received:
line. As another consequence of trace fields arising in
non-SMTP environments, receiving systems MUST NOT reject mail
based on the MAIL command, the
forward-path is the argument format of the RCPT command, a trace field and SHOULD be extremely
robust in the mail
data is the argument light of the DATA command. These arguments unexpected information or
data objects must be transmitted formats in
those fields.

The gateway SHOULD indicate the environment and held pending protocol in
the
confirmation communicated by "via" clauses of Received field(s) that it supplies.

3.8.3 From the end Internet side, the gateway SHOULD accept all valid
address formats in SMTP commands and in RFC-822 headers, and
all valid RFC-822 messages. Gateways are, of mail data indication
which finalizes course, subject
to the transaction. The model same rules for this is handling source routes as those
described for other SMTP systems in section ##3.3.

3.8.4 The gateway MUST ensure that
distinct buffers are provided to hold the types all header fields of data objects,
that is, there is a reverse-path buffer, a forward-path buffer,
and a mail data buffer. Specific commands cause information to message
that it forwards into the Internet meet the requirements for
Internet mail. In particular, all addresses in "From:",
"To:", "Cc:", etc., fields must be appended to a specific buffer, or cause one or more buffers transformed (if necessary)
to satisfy RFC-822 syntax, and they must be cleared.

4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)

These commands are effective and
useful for sending replies.

3.8.5 The translation algorithm used to identify convert mail from the SMTP client
Internet protocols to another environment's protocol SHOULD
ensure that error messages from the SMTP
server. The argument field contains foreign mail environment
are delivered to the fully-qualified domain name
of return path from the SMTP client if one is available. In situations envelope, not
to the sender listed in which the
SMTP client system does not have a meaningful domain name (e.g., when
its address is dynamically allocated and no reverse mapping record is
available, "From:" field of the client should send RFC-822
message.

3.8.6 Similarly, when forwarding a domain literal (see section
##__), optionally followed by information that will help to identify message from another environment
into the client system.

The SMTP server identifies itself to Internet, the SMTP client in gateway SHOULD set the
connection greeting reply, and envelope return
path in the response to this command.

A client SMTP SHOULD start accordance with an error message return address, if
any, supplied by the foreign environment.

3.9. Terminating Sessions and Connections

An SMTP session connection is terminated by issuing the EHLO client's sending a QUIT
command. If The server then responds with a positive reply code, after
which it closes the connection.

An SMTP server supports MUST NOT intentionally close the SMTP service extensions it
will give connection
except:
o After receiving a successful response, QUIT command and responding with a failure response, or an error
response. If 221
reply.
o After detecting the need to shutdown the SMTP server, in violation of this specification,
does not support any SMTP service extensions it will generate an
error response. Older client SMTP systems MAY, as discussed above,
use HELO (as specified in RFC 821) instead of EHLO.

These commands,
and returning a "250 OK" reply to one of them, confirm message with a 451 response code. This
response code can be issued after the server receives any
command or, if necessary, asynchronously from command
receipt (on the assumption that
both the SMTP client and will receive it
after the SMTP next command is issued).

In particular, a server are that closes connections in the initial state, response
to commands that is, there are not understood is no transaction in progress and all state tables and
buffers violation of this
specification. Instead, servers are cleared.

Normally, the response expected to EHLO will be a multiline reply. Each line tolerant of the response contains a keyword and, optionally, one or more
parameters. The syntax for
unknown commands, issuing a positive response, using the ABNF
notation 500 reply and low-level terminals of [ABNF], is:

ehlo-ok-rsp ::= "250" domain [ SP greeting ] CR LF
/ ( "250-" domain [ SP greeting ] CR LF
*( "250-" ehlo-line CR LF )
"250" SP ehlo-line CR LF )

; awaiting further
instructions from the usual HELO chit-chat
greeting ::= 1*<any character other than CR client.

An SMTP server which is forcibly shut down via external
means SHOULD attempt to send a line containing 451 response
code to the SMTP client before exiting. The SMTP client will
normally read the 451 response code after sending its next
command.

SMTP clients that experience a connection close, reset, or LF>

ehlo-line ::= ehlo-keyword *( SP ehlo-param )

ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")

; syntax other
communications failure due to circumstances not under their
control (in violation of the intent of this specification but
sometimes unavoidable) should, to maintain the robustness of the
mail system, treat the mail transaction as if a 451 response had
been received and values depend on ehlo-keyword
ehlo-param ::= 1*<any CHAR excluding SP act accordingly.

3.10 Mailing Lists and all
control characters (US ASCII 0-31
inclusive)>

[[xxx ALPHA ::= <any one Aliases

An SMTP-capable host SHOULD support both the alias and the list form
of address expansion for multiple delivery. When a message is
delivered or forwarded to each address of an expanded list form, the 52 alphabetic characters
(A through Z
return address in upper case, and, the envelope ("MAIL FROM:") MUST be changed to be
the address of a through z person who administers the list, but the message
header MUST be left unchanged; in lower case)>
DIGIT ::= <any one particular, the "From" field of the 10 numeric characters
(0 through 9)>

CR ::= <the carriage-return character
(ASCII decimal code 13)>
LF ::= <the line-feed character
(ASCII decimal code 10)>
SP ::= <the space character
(ASCII decimal code 32)> /xxx]]

Although EHLO keywords may be specified in upper, lower,
message is unaffected.

An important mail facility is a mechanism for multi-destination
delivery of a single message, by transforming or
mixed case, they must always be recognized and processed in "expanding" a
case-insensitive manner. This is simply an extension
pseudo-mailbox address into a list of
practices specified in RFC 821 and section ##2.4.1.

4.1.1.2 MAIL (MAIL)

This command destination mailbox addresses.
When a message is used sent to initiate such a mail transaction pseudo-mailbox (sometimes called an
"exploder"), copies are forwarded or redistributed to each mailbox in which
the mail
data is delivered to one or more mailboxes. The argument field
contains expanded list. We classify such a reverse-path.

The reverse-path consists of the sender mailbox pseudo-mailbox as an "alias"
or a list "list", depending upon the expansion rules:

3.10.1 Alias

To expand an alias, the recipient mailer simply replaces the
pseudo-mailbox address in the envelope with each of hosts as
described the expanded
addresses in Appendix C. In some types turn; the rest of reporting messages for
which a reply is likely to cause a mail loop (for example, mail
delivery the envelope and nondelivery notifications) the reverse-path message body are
left unchanged. The message is then delivered or forwarded to each
expanded address.

3.10.11 List

A mailing list may be null
(see section ##3.7).

This command clears said to operate by "redistribution" rather than
by "forwarding". To expand a list, the reverse-path buffer, recipient mailer replaces the forward-path buffer,
and
pseudo-mailbox address in the mail data buffer; and inserts envelope with each of the reverse-path information
from this command into expanded
addresses in turn. The return address in the reverse-path buffer.

If service extensions were negotiated, envelope is changed so
that all error messages generated by the MAIL command may also
carry parameters associated with final deliveries will be
returned to a particular service extension.

Syntax: "MAIL FROM:" Reverse-path [ SP Mail-parameters ]
or
"MAIL FROM:<>"

4.1.1.3 RECIPIENT (RCPT)

This command is used list administrator, not to identify an individual recipient the message originator, who
generally has no control over the contents of the list and will
typically find error messages annoying.

4. THE SMTP SPECIFICATIONS

4.1. SMTP COMMANDS

4.1.1. COMMAND SEMANTICS AND SYNTAX

The SMTP commands define the mail
data; multiple recipients transfer or the mail system
function requested by the user. SMTP commands are specified character strings
terminated by multiple use of this
command. <CRLF>. The forward-path normally consists a required destination mailbox.
Sending systems SHOULD not generate command codes themselves are alphabetic
characters terminated by <SP> if parameters follow and <CRLF>
otherwise. The syntax of the optimal list local part of hosts known
as a source route. Recieving systems MUST recognize source route mailbox must conform to
receiver site conventions and the syntax but SHOULD strip off specified in section
##4.1.2. The SMTP commands are discussed below. The SMTP replies
are discussed in Section ##4.2.

A mail transaction involves several data objects which are
communicated as arguments to different commands. The
reverse-path is the source route specification and
utilize argument of the domain name associated with MAIL command, the mailbox as if
forward-path is the source
route had not appeared.

Similarly, relay host SHOULD strip or ignore source routes argument of the RCPT command, and names
MUST NOT be copied into the reverse-path. When mail reaches its
ultimate destination (the forward-path contains only a destination
mailbox),
data is the SMTP server inserts it into argument of the destination mailbox in
accordance with its host mail conventions.

For example, mail received at relay host A with envelope commands

MAIL FROM:<USERX@HOSTY.ARPA>
RCPT TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>

will normally DATA command. These arguments or
data objects must be sent directly on to host D with envelope commands

MAIL FROM:<USERX@HOSTY.ARPA>
RCPT TO:<USERC@HOSTD.ARPA>

as provided in Appendix C, HostA MAY also choose to relay the
message to HostB, using the envelope commands

MAIL FROM:<USERX@HOSTY.ARPA>
RCPT TO:<@HOSTB.ARPA:USER@HOSTD.ARPA>

If service extensions were negotiated, the RCPT TO command may also
carry parameters associated with a particular service extension
offered by transmitted and held pending the server. The client MUST NOT transmit parameters other
than those associated with a service extension offered
confirmation communicated by the server
in its EHLO response.

Syntax: "RCPT TO:" Forward-path [ SP Rcpt-parameters ]

4.1.1.4 DATA (DATA)

The receiver treats the lines (strings ending in CRLF sequences, see
section ##2.3.7) following the command as end of mail data from indication
which finalizes the sender.
This command causes transaction. The model for this is that
distinct buffers are provided to hold the types of data objects,
that is, there is a reverse-path buffer, a forward-path buffer,
and a mail data from this command buffer. Specific commands cause information to
be appended to a specific buffer, or cause one or more buffers
to be cleared.

4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)

These commands are used to identify the mail data buffer. SMTP client to the SMTP
server. The mail data may contain any of argument field contains the 128 ASCII
character codes, although experience has indicated that use fully-qualified domain name
of
control characters other then SP, HT, CR, and LF may cause problems
and should be avoided when possible.

The mail data the SMTP client if one is terminated by a line containing only available. In situations in which the
SMTP client system does not have a period, that meaningful domain name (e.g., when
its address is dynamically allocated and no reverse mapping record is
available, the character sequence "<CRLF>.<CRLF>" client should send an address literal (see Section ##4.6.2 on
Transparency). This is section
##4.1.3), optionally followed by information that will help to
identify the end of mail data indication. client system.

The custom of accepting lines ending only in LF, as a concession SMTP server identifies itself to
non-conforming behavior on the part of some UNIX systems, has proven
to cause more interoperability problems than it solves and SMTP
server systems MUST NOT do this, even client in the name of improved
robustness. In particular,
connection greeting reply, and in the sequence "LF.LF" (bare line feeds,
without carriage returns) MUST NOT be treated as equivalent response to
CRLF.CRLF as the end of mail data indication.

Receipt of this command.

A client SMTP SHOULD start an SMTP session by issuing the end of mail data indication requires that EHLO
command. If the SMTP server
process supports the stored mail transaction information. This processing
consumes SMTP service extensions it
will give a successful response, a failure response, or an error
response. If the information SMTP server, in the reverse-path buffer, the forward-path
buffer, and the mail data buffer, and on the completion violation of this
command these buffers are cleared. If the processing is successful
the receiver must send an OK reply. If the processing fails
completely the receiver must send a failure reply. In sending a
positive completion reply to the end of data indication, the receiver
takes full responsibility for the message (see section ##6.1).

When the specification,
does not support any SMTP server accepts a message either for relaying or for
final delivery service extensions it inserts a trace record (also referred to
interchangabily will generate an
error response. Older client SMTP systems MAY, as a "time stamp line" or "Received" line) at the top
of the mail data. This trace record indicates the identity discussed above,
use HELO (as specified in RFC 821) instead of the
host that sent the message, EHLO.

These commands, and the identity a "250 OK" reply to one of the host that
received the message (and them, confirm that is inserting this time stamp), and
both the
date SMTP client and time the message was received. Relayed messages will have
multiple time stamp lines. Details for formation of these lines,
including their syntax, is specified SMTP server are in section ##4.4.

4.1.1.5 RESET (RSET)

This command specifies that the current mail transaction initial state,
that is, there is to be
aborted. Any stored sender, recipients, and mail data MUST be
discarded, no transaction in progress and all buffers and state tables and
buffers are cleared. The receiver
MUST send

Normally, the response to EHLO will be a "250 OK" multiline reply. A reset command may be issued by Each line
of the
client at any time. It is effectively equivalent to response contains a NOOP if issued
immediately after EHLO or HELO, or immediately before either of those
commands are issued. In other situations, it restores the state to
that immediately after the most recent EHLO keyword and, optionally, one or HELO. An SMTP server
MUST NOT close the connection as the result of receiving a RSET; that
action is reserved for QUIT (see section ##4.1.1.10, below).

Since EHLO and HELO imply some additional processing and response by
the server, RSET will normally be more efficient than reissuing those
commands even though the formal semantics are the same.

4.1.1.6 VERIFY (VRFY)

This command asks the receiver to confirm that the argument
identifies a user or mailbox. If it is
parameters. The syntax for a user name, positive response, using the full name ABNF
notation and low-level terminals of [ABNF], is:

ehlo-ok-rsp ::= "250" domain [ SP greeting ] CR LF
/ ( "250-" domain [ SP greeting ] CR LF
*( "250-" ehlo-line CR LF )
"250" SP ehlo-line CR LF )

; the user (if known) usual HELO chit-chat
greeting ::= 1*<any character other than CR or LF>

ehlo-line ::= ehlo-keyword *( SP ehlo-param )

ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")

; syntax and values depend on ehlo-keyword
ehlo-param ::= 1*<any CHAR excluding SP and all
control characters (US ASCII 0-31
inclusive)>

[[xxx ALPHA ::= <any one of the fully specified mailbox are returned.
VRFY is described 52 alphabetic characters
(A through Z in detail upper case, and,
a through z in section ##3.5.

This command has no effect on any lower case)>
DIGIT ::= <any one of the reverse-path buffer, the
forward-path buffer, or the mail data buffer.

Syntax: "VRFY" 10 numeric characters
(0 through 9)>

CR ::= <the carriage-return character
(ASCII decimal code 13)>
LF ::= <the line-feed character
(ASCII decimal code 10)>
SP String

4.1.1.7 EXPAND (EXPN) ::= <the space character
(ASCII decimal code 32)> /xxx]]

Although EHLO keywords may be specified in upper, lower, or
mixed case, they must always be recognized and processed in a
case-insensitive manner. This is simply an extension of
practices specified in RFC 821 and section ##2.4.1.

4.1.1.2 MAIL (MAIL)

This command asks the receiver is used to confirm that initiate a mail transaction in which the mail
data is delivered to one or more mailboxes. The argument
identifies field
contains a mailing list, and if so, to return the membership of
that list. reverse-path.

The full name reverse-path consists of the users (if known) and the fully
specified mailboxes are returned in sender mailbox or a multiline reply. EXPN is list of hosts as
described in detail in Appendix C. In some types of reporting messages for
which a reply is likely to cause a mail loop (for example, mail
delivery and nondelivery notifications) the reverse-path may be null
(see section ##3.5. ##3.7).

This command has no effect on any of clears the reverse-path buffer, the forward-path buffer, or
and the mail data buffer; and inserts the reverse-path information
from this command into the reverse-path buffer.

If service extensions were negotiated, the MAIL command may also
carry parameters associated with a particular service extension.

Syntax: "EXPN" "MAIL FROM:" Reverse-path [ SP String

4.1.1.8 HELP (HELP) Mail-parameters ]
or
"MAIL FROM:<>"

4.1.1.3 RECIPIENT (RCPT)

This command causes the receiver to send helpful information is used to identify an individual recipient of the
sender mail
data; multiple recipients are specified by multiple use of the HELP this
command.

The command MAY take an argument (e.g.,
any command name) and return more specific information as a response.

This command has no effect on any of the reverse-path buffer, the forward-path buffer, or the mail data buffer.

SMTP servers normally consists a required destination mailbox.
Sending systems SHOULD support HELP even if the form with an argument is
not supported.

Syntax: "HELP" [ SP String ]

4.1.1.9 NOOP (NOOP)

This command does not affect any parameters or previously entered
commands. It specifies no action other than that generate the receiver send
an OK reply.

This command has no effect on any optimal list of hosts known
as a source route. Receiving systems MUST recognize source route
syntax but SHOULD strip off the reverse-path buffer, source route specification and
utilize the
forward-path buffer, or domain name associated with the mail data buffer.

Syntax: "NOOP" [SP String]

4.1.1.10 QUIT (QUIT)

This command specifies that mailbox as if the receiver must send an OK reply, source
route had not appeared.

Similarly, relay hosts SHOULD strip or ignore source routes and
then close the transmission channel.

The receiver names
MUST NOT intentionally close be copied into the transmission channel
until it receives and replies to reverse-path. When mail reaches its
ultimate destination (the forward-path contains only a QUIT command (even if there was an
error). The sender MUST NOT intentionally close destination
mailbox), the transmission
channel until SMTP server inserts it send a QUIT command and receives the reply (even if
there was an error response to a previous command). If into the
connection is closed prematurely due destination mailbox in
accordance with its host mail conventions.

For example, mail received at relay host A with envelope commands

MAIL FROM:<USERX@HOSTY.ARPA>
RCPT TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>

will normally be sent directly on to violations host D with envelope commands

MAIL FROM:<USERX@HOSTY.ARPA>
RCPT TO:<USERC@HOSTD.ARPA>

as provided in Appendix C, HostA MAY also choose to relay the
message to HostB, using the envelope commands

MAIL FROM:<USERX@HOSTY.ARPA>
RCPT TO:<@HOSTB.ARPA:USER@HOSTD.ARPA>

of course, since hosts are not required to relay mail at all,
HostA may also reject the above or
system or network failure message entirely when the server MUST act as if RCPT TO
command is received, using a RSET 550 code (since this is a "policy
reason").

If service extensions were negotiated, the RCPT TO command
had been received (cancelling any pending transaction, but not
undoing any previously completed transaction) and may also
carry parameters associated with a particular service extension
offered by the server. The client MUST act
as if NOT transmit parameters other
than those associated with a service extension offered by the command or transaction server
in progress had received a temporary
error (4xx). its EHLO response.

Syntax: "QUIT"

4.1.2. LOWER-LEVEL SYNTAX "RCPT TO:" Forward-path [ SP Rcpt-parameters ]
or
"RCPT TO:<Postmaster>" [SP Rcpt-parameters ]

4.1.1.4 DATA (DATA)

The syntax of receiver treats the argument fields of lines (strings ending in CRLF sequences, see
section ##2.3.7) following the above commands (using command as mail data from the
syntax specified in [ABNF] where applicable) is given below. Some sender.
This command causes the mail data from this command to be appended to
the mail data buffer. The mail data may contain any of the productions given below are used only in conjunction with source
routes as described in Appendix C.

Reverse-path ::= Path

Forward-path ::= Path

Path ::= "<" [ A-d-l ":" ] <mailbox> ">"

A-d-l ::= At-domain *( "," A-d-l )

At-domain ::= "@" Domain

Mail-parameters ::= *( SP Keyword "=" Argument )

Rcpt-parameters ::= *( SP Keyword "=" Argument )

Keyword ::= String <<>>???
Argument ::= String <<>>???

Domain ::= sub-domain 1*("." sub-domain) | address-literal

sub-domain ::= let-dig *(ldh-str)
address-literal ::= "[" IPv4-address-literal |
IPv6-address-literal | General-address-literal "]"
IPv4-address-literal ::= snum 3*("." snum)
IPv6-address-literal ::= "IPv6" SP <<>>what did we finally decide on?
General-address-literal ::= Standardized-tag SP String
Standardized-tag ::= String (Specified in a standards-track RFC 128 ASCII
character codes, although experience has indicated that use of
control characters other then SP, HT, CR, and registered with IANA)
snum = one, two, or three digits representing LF may cause problems
and should be avoided when possible.

The mail data is terminated by a decimal
integer value in line containing only a period, that
is the range 0 through 255
let-dig = Alpha / Digit
ldh-str = *( Alpha / Digit / "-" ) 1*(let-dig)

Alpha = ASCII character in sequence "<CRLF>.<CRLF>" (see Section ##4.5.2 on
Transparency). This is the range A-Z or a-z. As specified in end of mail data indication. Note that
the domain name system definition [RFC-DNS], case first <CRLF> of this terminating sequence is not
significant in domain strings.
Digit = 0 - 9

Mailbox ::= Local-part "@" Domain

Local-part ::= Dot-string | Quoted-string

While also the definition for Local-part above is relatively permissive,
for maximum interoperability a host <CRLF> that expects to receive mail
SHOULD avoid defining mailboxes where
ends the Local-part requires (or
uses) final line of the Quoted-string form or where data (message text) or, if there was not
data, ends the Local-part is
case-sensitive. For any purposes that require generating or
comparing Local-parts (e.g., to specific mailbox names), all quoted
forms DATA command itself. An extra <CRLF> MUST NOT be treated
added, as equivalent and that would cause an empty line to be added to the sending system SHOULD
transmit message.
The only exception to this rule would arise if the form message body were
passed to the originating SMTP-sender with a final "line" that uses did
not end in <CRLF>; in that case, the minimum quoting possible.

Systems originating SMTP system MUST NOT define mailboxes in such a way
either reject the message as invalid or add <CRLF> in order to require have
the use receiving SMTP server recognize the "end of non-ASCII characters (octets with data" condition.

The custom of accepting lines ending only in LF, as a concession to
non-conforming behavior on the high order bit set part of some UNIX systems, has proven
to one)
or ASCII "control characters" (decimal value 0-31 cause more interoperability problems than it solves and 127). These
characters SMTP
server systems MUST NOT be used do this, even in MAIL FROM or RCPT TO commands or other
commands that require mailbox names.

<<?>> <string> ::= <char> | <char> <string>

<<?>> <quoted-string> ::= """ <qtext> """

<<?>> <qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>

<CR> ::= the carriage return character (ASCII code 13)

<LF> ::= the name of improved
robustness. In particular, the sequence "LF.LF" (bare line feed character (ASCII code 10)

<SP> ::= feeds,
without carriage returns) MUST NOT be treated as equivalent to
CRLF.CRLF as the space character (ASCII code 32)

<a> ::= any one end of the 52 alphabetic characters A through Z
in upper case and a through z in lower case

<c> ::= any one mail data indication.

Receipt of the 128 ASCII characters, but not any
<special> or <SP>

<d> ::= any one end of mail data indication requires that the ten digits 0 through 9

<q> ::= any one of server
process the 128 ASCII characters except <CR>,
<LF>, quote ("), or backslash (\)

<x> ::= any one of stored mail transaction information. This processing
consumes the 128 ASCII characters (no exceptions)

<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | information in the control
characters (ASCII codes 0 through 31 inclusive reverse-path buffer, the forward-path
buffer, and
127)

Note that the backslash, "\", is a quote character, which is
used to indicate that mail data buffer, and on the next character is to be used
literally (instead completion of its normal interpretation). For example,
"Joe\,Smith" could be used to indicate a single nine character
user field with comma being this
command these buffers are cleared. If the fourth character of processing is successful
the field.

Characters outside receiver must send an OK reply. If the set of specials, alphas, digits, and
hyphen MUST NOT appear in domain names. In particular, processing fails
completely the
underscore character is not permitted.

4.1.3. Address literals

Sometimes receiver must send a host is not known to the domain name system and
communication (and, in particular, communication failure reply. In sending a
positive completion reply to report and
repair the error) is blocked. To bypass this barrier a special
literal form end of data indication, the address is also allowed as an alternative receiver
takes full responsibility for the message (see section ##6.1).

When the SMTP server accepts a message either for relaying or for
final delivery it inserts a trace record (also referred to
host "names". For IPv4 addresses, this form uses four
interchangabily as a "time stamp line" or more
small decimal integers separated by dots and enclosed by
brackets, e.g., "[123.255.37.2]", which "Received" line) at the top
of the mail data. This trace record indicates an (IPv4)
Internet Address in sequence-of-octets form. For IPv6 and
other forms the identity of addressing the
host that might eventually be standardized, sent the form consists message, and the identity of a standardized "tag" the host that identifies
received the
address syntax, a space, message (and that is inserting this time stamp), and the address itself, in a format
specified elsewhere.

4.1.4. Order of commands

There are restrictions on
date and time the order in which message was received. Relayed messages will have
multiple time stamp lines. Details for formation of these commands may
be used.

A session lines,
including their syntax, is specified in section ##4.4.

4.1.1.5 RESET (RSET)

This command specifies that the current mail transaction is to contain be
aborted. Any stored sender, recipients, and mail transactions data MUST first be
initialized by the use of the HELO or EHLO command. An SMTP
server SHOULD accept commands for non-mail transactions (e.g.,
VRFY or EXPN) without this initialization.

HELO or EHLO commands MAY
discarded, and all buffers and state tables cleared. The receiver
MUST send a "250 OK" reply. A reset command may be issued by a client later in the
session. If either
client at any time. It is effectively equivalent to a NOOP if issued
immediately after EHLO or HELO, or before either of those commands
are issued. In other situations, it restores the session begins, state to that
immediately after the most recent EHLO or HELO. An SMTP server MUST clear all buffers and state
NOT close the connection as if an RSET command
had been issued. In other words, the sequence result of RSET followed
immediately by HELO receiving a RSET; that
action is redundant, but not harmful other reserved for QUIT (see section ##4.1.1.10, below).

Since EHLO and HELO imply some additional processing and response by
the server, RSET will normally be more efficient than in reissuing those
commands even though the performance cost of executing unnecessary commands.

If formal semantics are the HELO or EHLO commands same.

There are not acceptable circumstances, contrary to the SMTP server,
501, 500, or 502 failure replies MUST be returned as appropriate.
The intent of this
specification, in which an SMTP server must stay in the same state after transmitting these
replies as it was in before may receive an indication that
the HELO underlying TCP connection has been closed or EHLO were received.

RFC 1123 contains a discussion reset. To preserve
the robustness of arguments to HELO and
conditions under which the HELO command can mail system, SMTP servers should be rejected. In
particular, HELO (or EHLO) MUST NOT be rejected because the
client's putative name does not match some criteria established
by the server (e.g., verification of reverse DNS mapping).

The NOOP, HELP, EXPN, VRFY, prepared
for this condition and RSET commands can be used at any
time during a session, or without previously initializing should treat them as if a
session. SMTP servers SHOULD process these normally (i.e., not
return RSET, followed by a 503 code) even if no HELO or EHLO command has yet
QUIT, had been
received; clients SHOULD open a session with HELO or EHLO received before
sending these commands.

If the above rules are followed, connection disappeared.

4.1.1.6 VERIFY (VRFY)

This command asks the example in RFC 821 that
shows "550 access denied to you" in response receiver to an EXPN command
is essentially meaningless unless a HELO or EHLO command
preceeds confirm that the EXPN argument
identifies a user or mailbox. If it is a user name, the denial full name of access
the user (if known) and the fully specified mailbox are returned.
VRFY is based described in detail in section ##3.5.

This command has no effect on any of the
client's IP address.

The MAIL command (or reverse-path buffer, the obsolete SEND, SOML,
forward-path buffer, or SAML commands)
begins a the mail transaction. Once started, data buffer.

Syntax: "VRFY" SP String

4.1.1.7 EXPAND (EXPN)

This command asks the receiver to confirm that the argument
identifies a mail transaction
consists mailing list, and if so, to return the membership of one
that list. The full name of the transaction beginning commands, one or
more RCPT commands, users (if known) and the fully
specified mailboxes are returned in a DATA command, multiline reply. EXPN is
described in that order. A mail
transaction may be aborted by detail in section ##3.5.

This command has no effect on any of the RSET (or a new EHLO or HELO)
command. There may be zero reverse-path buffer, the
forward-path buffer, or more transactions in a session.

If the transaction beginning mail data buffer.

Syntax: "EXPN" SP String

4.1.1.8 HELP (HELP)

This command argument is not acceptable
a 501 failure reply MUST be returned and causes the SMTP server must
stay in receiver to send helpful information to the same state. If
sender of the commands in HELP command. The command MAY take an argument (e.g.,
any command name) and return more specific information as a transaction are
out response.

This command has no effect on any of order to the degree that they cannot be processed by reverse-path buffer, the
server a 503 failure reply MUST be returned and
forward-path buffer, or the mail data buffer.

SMTP server
must stay in the same state.

The last command in a session must be servers SHOULD support HELP even if the QUIT command. The
QUIT form with an argument is
not supported.

Syntax: "HELP" [ SP String ]

4.1.1.9 NOOP (NOOP)

This command can does not be used at affect any parameters or previously entered
commands. It specifies no action other time in a session, but
SHOULD be used by than that the client SMTP to request connection-closing
even if no session-opening receiver send
an OK reply.

This command has been sent and accepted.

4.1.5 Private-use commands

Commands starting in "X" may be used by bilateral agreement
between no effect on any of the client (sending) and server (receiving) SMTPs. An
SMTP server reverse-path buffer, the
forward-path buffer, or the mail data buffer.

Syntax: "NOOP" [SP String]

4.1.1.10 QUIT (QUIT)

This command specifies that does not recognize such the receiver must send an OK reply, and
then close the transmission channel.

The receiver MUST NOT intentionally close the transmission channel
until it receives and replies to a QUIT command is expected
to reply with "500 Command not recognized". An extended SMTP
server MAY list (even if there was an
error). The sender MUST NOT intentionally close the feature names associated with these private
commands in transmission
channel until it send a QUIT command and receives the reply (even if
there was an error response to a previous command). If the EHLO command.

Commands sent or accepted by SMTP systems that do not start with
"X" MUST be documented in published RFCs and be at least
candidates for standardization.

4.2. SMTP REPLIES

Replies to SMTP commands are devised
connection is closed prematurely due to ensure the synchronization violations of
requests and actions in the process of mail transfer, above or
system or network failure the server MUST act as if a RSET command
had been received (cancelling any pending transaction, but not
undoing any previously completed transaction) and to
guarantee that the SMTP client always knows the state of MUST act
as if the SMTP
server. Every command must generate exactly one reply. or transaction in progress had received a temporary
error (4xx).

Syntax: "QUIT"

4.1.2. LOWER-LEVEL SYNTAX

The details syntax of the command-reply sequence are made explicit in
Section ##4.3 on Sequencing.

An SMTP reply consists argument fields of a three digit number (transmitted as three
alphanumeric characters) followed by some text. The number is
intended for use by automata to determine what state to enter next; the text is meant for above commands (using the human user. It
syntax specified in [ABNF] where applicable) is intended that the three
digits contain enough encoded information that the SMTP client need
not examine the text and may either discard it or pass it on to the
user, as appropriate. In particular, the text may be
receiver-dependent and context dependent, so there are likely to be
varying texts for each reply code. A discussion given below. Some of
the theory of
reply codes is productions given below are used only in conjunction with source
routes as described in Appendix E. Formally, a reply is defined to
be the sequence: C.

Reverse-path ::= Path

Forward-path ::= Path

Path ::= "<" [ A-d-l ":" ] <mailbox> ">"

A-d-l ::= At-domain *( "," A-d-l )

At-domain ::= "@" Domain

Mail-parameters ::= *( SP Keyword "=" Argument )

Rcpt-parameters ::= *( SP Keyword "=" Argument )

Keyword ::= String <<>>???
Argument ::= String <<>>???

Domain ::= sub-domain 1*("." sub-domain) | address-literal

sub-domain ::= let-dig *(ldh-str)
address-literal ::= "[" IPv4-address-literal |
IPv6-address-literal | General-address-literal "]"
IPv4-address-literal ::= snum 3("." snum)
IPv6-address-literal ::= "IPv6" SP <<>>what did we finally decide on?
General-address-literal ::= Standardized-tag SP String
Standardized-tag ::= String (Specified in a three-digit code, SP, one line of text, standards-track RFC
and
CRLF, registered with IANA)
snum = one, two, or three digits representing a multiline reply (as defined decimal
integer value in Appendix E). Only the
EXPN and HELP commands are expected to result in multiline replies in
normal circumstances, however multiline replies range 0 through 255
let-dig = Alpha / Digit
ldh-str = *( Alpha / Digit / "-" ) let-dig

<<>> Placeholder: the following are allowed for any
command.

An SMTP server SHOULD send only not right, and the reply codes listed "alpha" one
may need separate rules for case-sensitive and case-insensitive.
Waiting for ABNF to stabilize. <<>>

Alpha = ASCII character in this
document. An SMTP server SHOULD use the text shown range A-Z or a-z. As specified in
the examples
whenever appropriate.

A client SMTP MUST determine its actions only by the reply
code, domain name system definition [RFC-DNS], case is not by
significant in domain strings.
Digit = 0 - 9

Mailbox ::= Local-part "@" Domain

Local-part ::= Dot-string | Quoted-string

While the text (except definition for 251 and 551 replies); any
text, including no text at all, must be acceptable. The space
(blank) following the reply code Local-part above is considered part of the
text. Whenever possible, relatively permissive,
for maximum interoperability a sender-SMTP host that expects to receive mail
SHOULD test only the
first digit of avoid defining mailboxes where the reply code.

4.2.1. REPLY CODES BY FUNCTION GROUPS

500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented (see section ##4.2.3)
503 Bad sequence of commands
504 Command parameter not implemented

211 System status, or system help reply
214 Help message
[Information on how to use Local-part requires (or
uses) the receiver Quoted-string form or where the meaning of a
particular non-standard command; this reply Local-part is useful only
to the human user]

220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available,
closing transmission channel
[This may be a reply to
case-sensitive. For any command if the service knows it
must shut down]

250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
[See section ##3.4]
252 Cannot VRFY user, but will accept message and attempt
delivery
[See section ##___]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access, purposes that require generating or command rejected
for policy reasons]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
[See section ##3.4]
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g.,
comparing Local-parts (e.g., to specific mailbox syntax incorrect]
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed

4.2.2. NUMERIC ORDER LIST OF REPLY CODES

211 System status, or names), all quoted
forms MUST be treated as equivalent and the sending system help reply
214 Help message
[Information on how to use SHOULD
transmit the receiver or form that uses the meaning of minimum quoting possible.

Systems MUST NOT define mailboxes in such a
particular non-standard command; this reply is useful only way as to require the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
[See section ##3.4]
252 Cannot VRFY user, but will accept message and attempt
delivery
[See section ##__]

354 Start mail input; end use
of non-ASCII characters (octets with <CRLF>.<CRLF>

421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage

500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error high order bit set to one)
or ASCII "control characters" (decimal value 0-31 and 127). These
characters MUST NOT be used in parameters MAIL FROM or arguments
502 Command not implemented
503 Bad sequence of RCPT TO commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access, or command rejected
for policy reasons]
551 User not local; please try <forward-path>
[See section ##3.4]
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., other
commands that require mailbox syntax incorrect]
554 Transaction failed

4.2.3. Reply names.

<<?>> <string> ::= <char> | <char> <string>

<<?>> <quoted-string> ::= """ <qtext> """

<<?>> <qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>

<CR> ::= the carriage return character (ASCII code 502

Questions have been raised as to when reply 13)

<LF> ::= the line feed character (ASCII code 502 (Command
not implemented) should be returned in preference to other
codes. 502 SHOULD be used when 10)

<SP> ::= the command is actually
recognized by space character (ASCII code 32)

<a> ::= any one of the SMTP server, 52 alphabetic characters A through Z
in upper case and a through z in lower case

<c> ::= any one of the 128 ASCII characters, but not implemented. If any
<special> or <SP>

<d> ::= any one of the
command is not recognized, code 500 SHOULD be returned.
Extended SMTP systems MUST NOT list capabilities in response to
EHLO for which they will return 502 (or 500) replies.

4.2.4 Reply ten digits 0 through 9

<q> ::= any one of the 128 ASCII characters except <CR>,
<LF>, quote ("), or backslash (\)

<x> ::= any one of the 128 ASCII characters (no exceptions)

<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | the control
characters (ASCII codes after DATA 0 through 31 inclusive and
127)

Note that the subsequent CRLF.CRLF.

When an SMTP server returns backslash, "\", is a positive completion status (2yz
code) after quote character, which is
used to indicate that the DATA command next character is completed to be used
literally (instead of its normal interpretation). For example,
"Joe\,Smith" could be used to indicate a single nine character
user field with CRLF.CRLF, it
accepts responsibity for:

+ delivering comma being the message (if fourth character of the recipient mailbox exists), or

+ if attempts to deliver field.

Characters outside the message fail due to transient
conditions, retrying delivery some reasonable number set of times
at intervals as specified alphas, digits, and hyphen MUST NOT
appear in RFC 1123, or

+ if attempts to deliver domain names. In particular, the message fail due to permanent
conditions, or if repeated attempts underscore character is
not permitted.

4.1.3. Address literals

Sometimes a host is not known to deliver the message
fail due to transient conditions, returning appropriate
notification domain name system and
communication (and, in particular, communication to report and
repair the sender error) is blocked. To bypass this barrier a special
literal form of the original message (using the address in the SMTP MAIL FROM command).

When an SMTP server returns a transient error completion status
(4yz) code after the DATA command is completed with CRLF.CRLF,
it MUST NOT make any further attempt also allowed as an alternative to deliver
host "names". For IPv4 addresses, this form uses four or more
small decimal integers separated by dots and enclosed by
brackets, e.g., "[123.255.37.2]", which indicates an (IPv4)
Internet Address in sequence-of-octets form. For IPv6 and
other forms of addressing that message.
The SMTP client retains responsibility for delivery might eventually be standardized,
the form consists of a standardized "tag" that
message identifies the
address syntax, a space, and may either return it to the user or requeue it for address itself, in a
subsequent attempt (see section ##4.5.4.1). The sending user
should format
specified elsewhere.

4.1.4. Order of commands

There are restrictions on the order in which these commands may
be able used.

A session that is to interpret contain mail transactions MUST first be
initialized by the return use of a transient the HELO or
permanent failure status as EHLO command. An SMTP
server SHOULD accept commands for non-mail transactions (e.g.,
VRFY or EXPN) without this initialization.

HELO or EHLO commands MAY be issued by a non-delivery indication.

4.3. SEQUENCING OF COMMANDS AND REPLIES

4.3.1 Sequencing overview

The communication between client later in the sender and receiver session.
If either is intended to be
an alternating dialogue, controlled by issued after the sender. As such, session begins, the
sender issues a command SMTP server MUST
clear all buffers and state as if an RSET command had been issued.
In other words, the receiver responds with a reply.
Unless sequence of RSET followed immediately by EHLO (or
HELO) is redundant, but not harmful other arrangements are negotiated through service extensions, than in the sender must wait for this response before sending further performance
cost of executing unnecessary commands.

One important reply is

If the connection greeting. Normally, a receiver
will send a 220 "Service ready" reply when HELO or EHLO commands are not acceptable to the connection is
completed. SMTP server,
501, 500, or 502 failure replies MUST be returned as appropriate.
The sender should wait for this greeting message before
sending any commands.

Note: all SMTP server must stay in the greeting type same state after transmitting these
replies have the official name (i.e., the
fully-qualified primary domain name) of the server host as it was in before the first
word following HELO or EHLO were received.

The SMTP client MUST ensure that the reply code. Sometimes Domain parameter to the EHLO or
HELO command is a valid principal host will have no
meaningful name. See ##4.1.3 for name (not a discussion of alternatives
in these situations.

For example,

220 ISIF.USC.EDU Service ready CNAME or

220 Loser.bogus.com Trashmail v 6.1.2 Service ready

The table below lists alternative success and failure replies MX name)
for
each command. These SHOULD be strictly adhered to; a receiver may
substitute text in the replies, but the meaning and action implied
by the code numbers and by the specific command reply sequence
cannot be altered.

COMMAND-REPLY SEQUENCES

Each command is listed with its usual host. If this is not possible replies. The prefixes
used before (e.g., when the possible replies are "P" for preliminary (not used in
SMTP), "I" for intermediate, "S" for success, "F" for failure, client's
address is dynamically assigned and
"E" for error. The 421 reply (service the client does not available, closing
transmission channel) may have an
obvious name), an address literal SHOULD be given to any command if substituted for the
SMTP-receiver knows it must shut down. Since some servers may
generate other replies under special circumstances,
Domain name and to allow for
future extension, supplemental information provided that will assist in
identifying the client.

An SMTP clients SHOULD, when possible, interpret only server MAY verify that the first digit of domain name parameter in the reply and MUST be prepared EHLO
or HELO command actually corresponds to deal with
unrecognized reply codes by interpreting the first digit only. SMTP
servers IP address of the client.
However, the server MUST NOT transmit reply codes refuse to an SMTP client that are
other than three digits or that do not start in accept a digit between 2 message if the
verification fails -- the information about verification failure is
for logging and
5 inclusive.

CONNECTION ESTABLISHMENT
S: 220
F: 421 tracing only.

The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any
time during a session, or without previously initializing a
session. SMTP servers SHOULD process these normally (i.e., not
return a 503 code) even if no HELO or EHLO (or HELO)
S: 250
E: 500*, 501, 504, 421, 550 command has yet been
received; clients SHOULD open a session with HELO or EHLO before
sending these commands.

If the above rules are followed, the example in RFC 821 that
shows "550 access denied to you" in response to an EXPN command
is essentially meaningless unless a HELO or EHLO command
preceeds the EXPN or the denial of access is based on the
client's IP address.

The MAIL
S: 250
F: 552, 451, 452
E: 500*, 501, 421, 550
RCPT
S: 250, 251 (but see section ##3.4 for discussion command (or the obsolete SEND, SOML, or SAML commands)
begins a mail transaction. Once started, a mail transaction
consists of 251)
F: 550, 551, 552, 553, 450, 451, 452
E: 500*, 501, 503, 421, 550 one of the transaction beginning commands, one or
more RCPT commands, and a DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500*, 501, 503, 421 command, in that order. A mail
transaction may be aborted by the RSET
S: 250
E: 500*, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250, 251, 252
F: 550, 551, 553
E: 500*, 501, 502, 504, 421
EXPN
S: 250, 252
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500*, 421
QUIT
S: 221
E: 500*
TURN
S: 250
F: 502
E: 500, 503

* Since support of this (or a new EHLO or HELO)
command. There may be zero or more transactions in a session.

If the transaction beginning command argument is required, returning
this not acceptable
a 501 failure reply code as part of an "unrecognized command"
status places an implementation MUST be returned and the SMTP server must
stay in the same state. If the commands in a transaction are
out of conformance with
this specification.

4.4 Trace information

When an SMTP order to the degree that they cannot be processed by the
server receives a message for delivery or further
processing, it 503 failure reply MUST insert trace ("time stamp" or "Received")
information at the beginning of be returned and the message content, as
discussed under SMTP server
must stay in the DATA same state.

The last command in section ##4.1.1.4.

This line a session must be structured as follows:

* The FROM field SHOULD contain both (1) the name of the
source host as presented QUIT command. The
QUIT command can not be used at any other time in a session, but
SHOULD be used by the EHLO or HELO client SMTP to request connection-closing
even if no session-opening command has been sent and (2)
an address literal containing the IP address of the source,
determined from the TCP connection.

* The ID field MAY contain an "@" as suggested accepted.

4.1.5 Private-use commands

Commands starting in RFC-822,
but this "X" may be used by bilateral agreement
between the client (sending) and server (receiving) SMTPs. An
SMTP server that does not recognize such a command is expected
to reply with "500 Command not required.

* The FOR field recognized". An extended SMTP
server MAY contain a list of <path> entries when
multiple RCPT the feature names associated with these private
commands have been given.

An Internet mail program MUST NOT change a Received: line that
was previously added to in the message header. SMTP servers MUST
prepend Received lines response to messages; they MUST NOT change the
order of existing lines EHLO command.

Commands sent or insert Received lines accepted by SMTP systems that do not start with
"X" MUST be documented in any other
location.

As the Internet grows, comparability of Received fields is
important published RFCs and be at least
candidates for detecting problems, especially slow relays. standardization.

4.2. SMTP
servers that create Received fields SHOULD use explicit offsets REPLIES

Replies to SMTP commands are devised to ensure the synchronization of
requests and actions in the dates (e.g., -0800), rather than time zone names process of any
type. Local time (with an offset) is preferred to UT when
feasible. If it is desired mail transfer, and to also use a time zone name, it
should be included in a commment.

When
guarantee that the delivery SMTP server makes client always knows the "final delivery" state of a
message, it inserts a return-path line at the beginning SMTP
server. Every command must generate exactly one reply.

The details of the
mail data. This use command-reply sequence are made explicit in
Section ##4.3 on Sequencing.

An SMTP reply consists of return-path is required; mail systems
MUST support it. a three digit number (transmitted as three
alphanumeric characters) followed by some text. The return path line preserves the information
in number is
intended for use by automata to determine what state to enter next;
the <reverse-path> from text is meant for the MAIL command. Here, final
delivery means human user. It is generally intended that
the message leaves three digits contain enough encoded information that the SMTP world. Normally,
this would mean
client need not examine the text and may either discard it has been delivered or pass it
on to the destination user or
an associated mail drop, but user, as appropriate. Exceptions are as noted in some cases it may the text,
in particular, the 220, 221, 251, 421, and 551 reply codes are
associated with message text that must be further
processed parsed and transmitted interpreted by another mail system.

It is possible for
machines. In the mailbox in general case, the return path to text may be
different from the actual sender's mailbox, for example if error
responses receiver-dependent
and context dependent, so there are likely to be delivered a special error handling mailbox
rather than to that varying texts for
each reply code. A discussion of the message sender. When mailing lists
are involved, this arrangement theory of reply codes is common and useful as given
in Appendix E. Formally, a means
of directing errors reply is defined to be the list maintainer rather than the
message originator.

The text above implies that the final mail data will begin with sequence: a return path line, followed by
three-digit code, SP, one or more time stamp lines.
These lines will be followed by the mail data headers line of text, and body
[RFC822]. See Example 8.

It is sometimes difficult for an SMTP server to determine
whether or not it is making final delivery since forwarding or
other operations may occur after the message is accepted for
delivery. Consequently, any further (forwarding, gateway, CRLF, or
relay) systems MAY remove a multiline
reply (as defined in Appendix E). Only the return path EXPN and rebuild the MAIL
FROM command as needed HELP commands
are expected to ensure that exactly one such line
appears result in a delivered message.

A message-originating multiline replies in normal circumstances,
however multiline replies are allowed for any command.

An SMTP system server SHOULD NOT send a message that
already contains a Return-path header. If a message that
contains more than one Return-path header is received, only the
first Return-path header line reply codes listed in this
document. An SMTP server SHOULD use the text shown in the message header is valid. examples
whenever appropriate.

A
message header processor SHOULD discard or, client SMTP MUST determine its actions only by the reply code, not
by the text (except for 251 and 551 and, if necessary just
ignore, necessary, 220, 221, and
421 replies); in the general case, any Return-path headers text, including no text at all
(although senders SHOULD NOT send bare codes), must be acceptable.
The space (blank) following the first one. But
such processors are normally not reply code is considered part of the mail transport
system; SMTP servers performing
text. Whenever possible, a relay function MUST NOT
inspect sender-SMTP SHOULD test only the message data to first
digit of the extent needed to determine if
Return-path headers are present. reply code.

The primary intent list of the Return-path is codes that it designates the
address to which messages indicating non-delivery or other mail
system failures at to be sent. For this to be unambigious,
exactly one return path should appears below must not be present when the message is
delivered. Systems using RFC 822 syntax with non-SMTP
transports SHOULD preserve construed as
permanent. While the intent addition of having an unambiguous
address, associated new codes should be a rare and
significant activity, with the transport envelope, to which to
send error reports (e.g., non-delivery messages).

Historical note: Text supplemental information in RFC 822 that appears to contradict the
use textual
part of Return-path (or the envelope MAIL FROM address) response being preferred, new codes may from time to time
be added as the
destination result of error messages is new Standards or Standards-track
specifications. Consequently, a sender-SMTP MUST be prepared to
handle codes not applicable on specified in this document and MUST do so by
interpreting the Internet. first digit (severity indication) only.

4.2.1. REPLY CODE SEVERITIES AND THEORY

The MAIL FROM address (as copied into three digits of the Return-path) MUST reply each have a special significance.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated SMTP client, or one that
receives an unexpected code, will be
used able to determine its next
action (proceed as the target planned, redo, retrench, etc.) by simply
examining this first digit. An SMTP client that wants to know
approximately what kind of any mail containing delivery error messages.

In particular,

(i) a gateway from SMTP->elsewhere SHOULD insert a return-path
header, unless it is known that the "elsewhere" transport
also uses Internet domain addresses and maintains occurred (e.g., mail system
error, command syntax error) may examine the
envelope sender address separately.

(ii) a gateway from elsewhere->SMTP SHOULD delete any
return-path header present in second digit,
reserving the message, third digit and either copy
that information to the SMTP envelope or combine it with any supplemental information that
may be present in for the envelope finest gradation of information.

There are five values for the other transport
system to construct the MAIL FROM part first digit of the SMTP envelope.

Special mention is needed of reply code:

1yz Positive Preliminary reply

The command has been accepted, but the response and further requested action required
when the processing following the end of mail data indication
is
partially successful. This could arise if after accepting several
recipients and the mail data, the SMTP server finds that the mail
data can be successfully delivered to some being held in abeyance, pending confirmation of the recipients, but it
cannot be to others (for example, due to mailbox space allocation
problems). In such a situation, the response to the DATA command
must be an OK
information in this reply. But, the The SMTP server must compose and client should send an
"undeliverable mail" notification message
another command specifying whether to continue or abort
the originator of the
message. Either a single notification which lists all of the
recipients action.

[Note: SMTP does not have any commands that failed to get allow this
type of reply, and so does not have the message, continue or separate notification
messages must be sent for each failed recipient (see Example 7). All
undeliverable mail notification messages are sent using the MAIL
abort commands.]

2yz Positive Completion reply

The requested action has been successfully completed. A
new request may be initiated.

3yz Positive Intermediate reply

The command (even if they result from processing has been accepted, but the obsolete SEND, SOML,
or SAML commands) and use a null return path as discussed requested action
is being held in
section ##3.7.

<<<>>> abeyance, pending receipt of further
information. The following section SMTP client should send another command
specifying this information. This reply is incomplete used in this draft <<>>
command sequence groups.

4yz Transient Negative Completion reply

The time stamp line command was not accepted and the return path line are formally defined as
follows:

<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>

<time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>

<from-domain> ::= "FROM" <SP> <domain> <SP>

<by-domain> ::= "BY" <SP> <domain> <SP>

<opt-info> ::= [<via>] [<with>] [<id>] [<for>]

<<>>FOR requested action did
not occur. However, the error condition is temporary and <link> need to be nailed down.

<protocol> ::= action may be requested again. The standard names for protocols are
registered with sender should
return to the Internet Assigned Numbers Authority
(IANA).

Note that beginning of the earlier form, which permits
two-digit years, is deprecated. SMTP systems
SHOULD use four-digit years.

<dd> ::= the one or command sequence (if any).
It is difficult to assign a meaning to "transient" when
two decimal integer day of different sites (receiver- and sender- SMTPs) must
agree on the month interpretation. Each reply in this category
might have a different time value, but the range 1 SMTP client is
encouraged to 31.

<mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"

<yyyy> ::= try again. A rule of thumb to determine if
a reply fits into the four decimal integer year 4yz or the 5yz category (see below)
is that replies are 4yz if they can be repeated without
any change in command form or in properties of the range 0000 to
9999.

<hh> ::= sender
or receiver. (E.g., the two decimal integer hour of command is repeated identically
and the day in receiver does not put up a new implementation.)

5yz Permanent Negative Completion reply

The command was not accepted and the
range 00 to 24.

<mm> ::= requested action did
not occur. The SMTP client is discouraged from repeating
the two decimal integer minute of exact request (in the hour in same sequence). Even some
"permanent" error conditions can be corrected, so the
range 00
human user may want to 59.

<ss> ::= direct the two decimal integer second of SMTP client to
reinitiate the minute command sequence by direct action at some
point in the
range 00 to 59.

<zone> ::= A four-digit time zone offset, such as -0600 for US
Eastern Standard Time. This may be supplemented by a
time zone name future (e.g., after the spelling has been
changed, or the user has altered the account status).

The second digit encodes responses in parentheses, e.g., "-0800 (PDT)". See
??? for additional discussion.

Note specific categories:

x0z Syntax -- These replies refer to syntax errors,
syntactically correct commands that there is no default; time zone information
is required and MUST be supplied.

-------------------------------------------------------------
|
| Example of Return Path don't fit any
functional category, and Received Time Stamps
|
| Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
| Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 -0800
| Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 -0800
| Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 -0800
| Date: 27 Oct 81 15:01:01 -0800 (PST)
| From: JOE@ABC.ARPA
| Subject: Improved Mailing System Installed
| To: SAM@JKL.ARPA
|
| This is unimplemented or superfluous
commands.

x1z Information -- These are replies to inform you that ...
|
| Example 8
|
-------------------------------------------------------------

4.5. DETAILS

4.5.1. MINIMUM IMPLEMENTATION

In order requests for
information, such as status or help.

x2z Connections -- These are replies referring to make SMTP workable, the following minimum
implementation is required for all receivers:

COMMANDS
transmission channel.

x3z Unspecified as yet.

x4z Unspecified as yet.

x5z Mail system -- HELO
VRFY
MAIL
RCPT
DATA
RSET
NOOP
QUIT

Any These replies indicate the status of
the receiver mail system that includes an SMTP server that supports RCPT MUST
support the reserved mailbox "Postmaster" as a case-insensitive
mailbox name. EHLO SHOULD be supported if possible.

4.5.2. TRANSPARENCY

Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends vis-a-vis the requested
transfer or other mail text and cannot be sent system action.

The third digit gives a finer gradation of meaning in each
category specified by the user. In general, users are not aware second digit. The list of such
"forbidden" sequences. To allow all user composed replies
illustrates this. Each reply text is recommended rather than
mandatory, and may even change according to be
transmitted transparently the following procedures are used.

1. Before sending a line of mail text the SMTP client checks the first character of the line. If command with
which it is a period, one
additional period is inserted at associated. On the beginning of other hand, the line.

2. When a line of mail text is received by reply codes
must strictly follow the SMTP server
it checks specifications in this section.
Receiver implementations should not invent new codes for
slightly different situations from the line. If ones described here, but
rather adapt codes already defined.

For example, a command such as NOOP whose successful execution
does not offer the line is composed of SMTP client any new information will return
a single
period it 250 reply. The response is 502 when the end command requests an
unimplemented non-site-specific action. A refinement of mail. If the first character that
is a
period and there are other characters on the line, the first
character 504 reply for a command that is deleted. implemented, but that
requests an unimplemented parameter.

The mail data reply text may contain any of the 128 ASCII characters. All
characters are to be delivered to longer than a single line; in these cases
the recipient's mailbox
including complete text must be marked so the SMTP client knows when it
can stop reading the reply. This requires a special format effectors and other control characters. If to
indicate a multiple line reply.

The format for multiline replies requires that every line,
except the transmission channel provides an 8-bit byte (octets) data
stream, last, begin with the 7-bit ASCII codes are transmitted right justified
in reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with the octets reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.

For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line

In many cases the high order bits cleared SMTP client then simply needs to zero. See
##___ search for special treatment
the reply code followed by <SP> at the beginning of these conditions in SMTP systems
serving a relay function.

In some systems it may be necessary to transform the data as
it is received line, and stored. This may be necessary for hosts
that use
ignore all preceding lines. In a different character set than ASCII as their local
character set, or that store few cases, there is important
data for the sender in records rather than
strings. If such transforms are necessary, they must be
reversible -- especially if the reply "text". The sender will know
these cases from the current context.

4.2.2. REPLY CODES BY FUNCTION GROUPS

500 Syntax error, command unrecognized
[This may include errors such transforms are applied to
mail being relayed.

4.5.3. SIZES AND TIMEOUTS

There are several objects that have required minimum maximum
sizes. That is, every implementation must be able to receive
objects of at least these sizes, but SHOULD NOT send objects
larger than these sizes. In particular, note that, as command line too long]
501 Syntax error in some
cases, the sizes given may parameters or arguments
502 Command not be sufficient to accomodate
important practices and protocols. For example, the length
specified for a "user name" (local-part) may implemented (see section ##4.2.3)
503 Bad sequence of commands
504 Command parameter not be sufficient implemented

211 System status, or system help reply
214 Help message
[Information on how to accomodate an encoded X.400 address [RFC-X400].

****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************

user

The maximum total length of a user name is 64 characters.

domain

The maximum total length of a domain name use the receiver or number is 64
characters.

path

The maximum total length the meaning of a reverse-path or
forward-path
particular non-standard command; this reply is 256 characters (including useful only
to the punctuation
and element separators).

command line

The maximum total length of human user]

220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command line including the
command word and if the <CRLF> is 512 characters.

reply line

The maximum total length of a reply line including the
reply code service knows it
must shut down]

250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
[See section ##3.4]
252 Cannot VRFY user, but will accept message and the <CRLF> is 512 characters.

text line

The maximum total length of a text line including the
<CRLF> is 1000 characters (but attempt
delivery
[See section ##3.5.3]
450 Requested mail action not counting the leading
dot duplicated taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access, or command rejected
for transparency). This number may be increased by policy reasons]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
[See section ##3.4]
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed [Or, in the use of SMTP Service Extensions.

message content

The maximum total length case of a connection-opening
response, "No SMTP service here"]

4.2.3. NUMERIC ORDER LIST OF REPLY CODES

211 System status, or system help reply
214 Help message content (including any
message headers as well as
[Information on how to use the message body) MUST BE at least 64K
octets. Especially since receiver or the introduction meaning of multimedia mail
[RFC-MIME], message lengths on a
particular non-standard command; this reply is useful only
to the Internet have grown
dramatically, and message size restrictions should be avoided
if at all possible. SMTP server systems that must impose
restrictions SHOULD implement the "SIZE" service extension
([RFC-SIZE]) and SMTP client systems that human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will send large
messages SHOULD utilize it when possible.

recipients buffer

The maximum total number of recipients that must be
buffered is 100 recipients.

Errors due forward to exceeding these limits <forward-path>
[See section ##3.4]
252 Cannot VRFY user, but will accept message and attempt
delivery
[See section ##3.5.3]

354 Start mail input; end with <CRLF>.<CRLF>

421 <domain> Service not available,
closing transmission channel
[This may be reported by using
the a reply codes, for example: to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage

500 Line Syntax error, command unrecognized
[This may include errors such as command line too long. long]
501 Path too long

552 Too many recipients. Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access, or command rejected
for policy reasons]
551 User not local; please try <forward-path>
[See section ##3.4]
552 Too much Requested mail data.

An SMTP client should provide timeouts for all commands. Minimum
values SHOULD be as follows:

o Initial 220 Message: 5 minutes

An SMTP client process needs to distinguish between a action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
554 Transaction failed TCP connection and a delay [Or, in receiving the initial
220 greeting message. Many SMTP servers will accept a
TCP connection but delay delivery case of the 220 message until
their system load will permit more mail a connection-opening
response, "No SMTP service here"]

4.2.4. Reply code 502

Questions have been raised as to when reply code 502 (Command
not implemented) should be processed.

o MAIL Command: 5 minutes

o RCPT Command: 5 minutes

A longer timeout would returned in preference to other
codes. 502 SHOULD be required if processing of
mailing lists and aliases were not deferred until after used when the message was accepted.

o DATA Initiation: 2 minutes

This command is while awaiting actually
recognized by the "354 Start Input" reply SMTP server, but not implemented. If the
command is not recognized, code 500 SHOULD be returned.
Extended SMTP systems MUST NOT list capabilities in response to a
EHLO for which they will return 502 (or 500) replies.

4.2.5 Reply codes after DATA command.

o Data Block: 3 minutes

This is while awaiting and the completion of each TCP SEND
call transmitting subsequent CRLF.CRLF.

When an SMTP server returns a chunk of data.

o positive completion status (2yz
code) after the DATA Termination: 10 minutes.

This command is while awaiting the "250 OK" reply. When the
receiver gets the final period terminating completed with CRLF.CRLF, it
accepts responsibity for:

+ delivering the message
data, it typically performs processing (if the recipient mailbox exists), or

+ if attempts to deliver the message fail due to a user mailbox. A spurious timeout at this
point would be very wasteful and would typically result in transient
conditions, retrying delivery some reasonable number of multiple copies times
at intervals as specified in section ##4.2.6.

+ if attempts to deliver the message fail due to permanent
conditions, or if repeated attempts to deliver the message
fail due to transient conditions, returning appropriate
notification to the sender of the message, since it has been
successfully sent and original message (using the server has accepted
responsibility for delivery. See section ##6.1 for
additional discussion.

An
address in the SMTP MAIL FROM command).

When an SMTP server SHOULD have returns a timeout of at least 5 minutes
while it is awaiting transient error completion status
(4yz) code after the next DATA command from the sender.

4.5.4 Queuing Strategies is completed with CRLF.CRLF,
it MUST NOT make any further attempt to deliver that message.
The common structure of a host SMTP implementation includes
user mailboxes, one or more areas client retains responsibility for queueing messages in
transit, delivery of that
message and one may either return it to the user or more daemon processes requeue it for sending and
receiving mail. a
subsequent attempt (see section ##4.5.4.1). The exact structure will vary depending on sending user
should be able to interpret the
needs return of a transient or
permanent failure status as a non-delivery indication.

4.3. SEQUENCING OF COMMANDS AND REPLIES

4.3.1 Sequencing overview

The communication between the users on the host sender and receiver is intended to be
an alternating dialogue, controlled by the number and size of
mailing lists supported by sender. As such, the host. We describe several
optimizations that have proved helpful, particularly for
mailers supporting high traffic levels.

Any queueing strategy MUST include:

o Timeouts on all activities on a per-command basis

o Never sending error messages in response to error messages.

4.5.4.1 Sending Strategy

The general model of an SMTP client is one or more processes
that periodically attempt to transmit outgoing mail. In
sender issues a
typical system, command and the program that composes receiver responds with a message has some
method for requesting immediate attention reply.
Unless other arrangements are negotiated through service extensions,
the sender must wait for a new piece of
outgoing mail, while mail that cannot be transmitted
immediately MUST be queued and periodically retried by this response before sending further
commands.

One important reply is the
sender. A mail queue entry connection greeting. Normally, a receiver
will include not only the
message itself but also send a 220 "Service ready" reply when the envelope information. connection is
completed. The sender MUST delay retrying a particular destination
after one attempt has failed. In general, should wait for this greeting message before
sending any commands.

Note: all the retry
interval SHOULD be at least 30 minutes; however, more
sophisticated and variable strategies will be beneficial
when greeting type replies have the SMTP client can determine official name (i.e., the reason for non-
delivery.

Retries continue until
fully-qualified primary domain name) of the message is transmitted or server host as the
sender gives up; first
word following the give-up time generally needs to be at
least 4-5 days. The parameters to reply code. Sometimes the retry algorithm MUST
be configurable.

A sender SHOULD keep host will have no
meaningful name. See ##4.1.3 for a list discussion of hosts it cannot reach alternatives
in these situations.

For example,

220 ISIF.USC.EDU Service ready
or

220 Loser.bogus.com Trashmail v 6.1.2 Service ready

The table below lists alternative success and
corresponding connection timeouts, rather than just retrying queued
mail items.

DISCUSSION:
Experience suggests that failures are typically
transient (the target system has crashed), favoring failure replies for
each command. These SHOULD be strictly adhered to; a
policy of two connection attempts receiver may
substitute text in the first hour the
message is in replies, but the queue, meaning and then backing off to once
every two or three hours.

The SMTP client can shorten action implied
by the queueing delay code numbers and by
cooperation with the SMTP server. In particular, if
mail is received from a particular address, it is good
evidence that any mail queued for that host can now specific command reply sequence
cannot be
sent. Application of this principle may, in many cases,
eliminate altered.

4.3.2 Command-Reply Sequences

Each command is listed with its usual possible replies. The prefixes
used before the requirement possible replies are "P" for an explicit "send queues now"
function such as that discussed preliminary (not used in [RFC-ETRN].
SMTP), "I" for intermediate, "S" for success, "F" for failure, and
"E" for error. The strategy 421 reply (service not available, closing
transmission channel) may be further modified as a result of
multiple addresses per host (see below), given to optimize
delivery time vs. resource usage.

An SMTP client may have a large queue of messages for
each unavailable destination host. If any command if the
SMTP-receiver knows it retried all of
these messages in every retry cycle, there would be
excessive Internet overhead must shut down. Since some servers may
generate other replies under special circumstances, and the daemon would be
blocked to allow for a long period. Note that an
future extension, SMTP client can
generally determine that a delivery attempt has failed clients SHOULD, when possible, interpret only after a timeout of several minutes; even a one minute
timeout per connection will result in a very large
delay if it is repeated for dozens or even hundreds of
queued messages to the same host.

At
the same time, SMTP clients should use great care about
caching negative responses from servers. In an extreme
case, if EHLO is issued multiple times during first digit of the same SMTP
connection, different answers may reply and MUST be returned prepared to deal with
unrecognized reply codes by interpreting the
server. More significantly, 5yz responses to MAIL FROM first digit only. SMTP
servers MUST NOT be cached.

When the same message is to be delivered transmit reply codes to several users on
the same host, only one copy of the message SHOULD be
transmitted. That is, the SMTP client SHOULD use the
command sequence: RCPT, RCPT,... RCPT, DATA instead of the
sequence: RCPT, DATA, ..., RCPT, DATA, ... RCPT, DATA.
Implementation of this efficiency feature is strongly urged.

Similarly, the an SMTP client MAY support multiple concurrent
outgoing mail transactions to achieve timely delivery.
However, some limit may be appropriate to protect the host
from devoting all its resources to mail.

4.5.4.2 Receiving strategy

The SMTP server SHOULD attempt to keep a pending listen on
the SMTP port at all times. This will require the support
of multiple incoming TCP connections for SMTP. Some limit
MAY be imposed.

IMPLEMENTATION:
When the SMTP server receives mail from a particular
host address, it could notify the SMTP client to retry
any mail pending for that host address.

5. Address resolution and mail handling

Once an SMTP client lexically identifies a domain to which mail is to
be delivered for processing (as described in sections ##3.6 and
##3.7), a DNS lookup is performed to resolve that domain name (see
[RFC-DNS]). The lookup first attempts to locate an MX record
associated with that name. If a CNAME record is found instead, the
resulting name is processed as if it were the initial name. If no MX
records are found, but an A RR is found, the A RR is treated as if it
was associated with an implicit MX RR, with a preference of 0,
pointing to that host.

When the lookup succeeds, the mapping can result in a list of
alternative delivery addresses rather
other than a single address, because
of (a) multiple MX records, (b) multihoming, three digits or both. To provide
reliable mail transmission, the SMTP client MUST be able to try (and
retry) each of the relevant addresses in this list that do not start in order, until a
delivery attempt succeeds. However, there MAY also be a configurable
limit on the number of alternate addresses that can be tried. In any
case, digit between 2 and
5 inclusive.

CONNECTION ESTABLISHMENT
S: 220
F: 421, 554
EHLO (or HELO)
S: 250
E: 500*, 501, 504, 421, 550
MAIL
S: 250
F: 552, 451, 452
E: 500*, 501, 421, 550, 553
RCPT
S: 250, 251 (but see section ##3.4 for discussion of 251)
F: 550, 551, 552, 553, 450, 451, 452
E: 500*, 501, 503, 421, 550
DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500*, 501, 503, 421
RSET
S: 250
E: 500*, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250, 251, 252
F: 550, 551, 553
E: 500*, 501, 502, 504, 421
EXPN
S: 250, 252
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500*, 421
QUIT
S: 221
E: 500*
TURN
S: 250
F: 502
E: 500, 503

* Since support of this command is required, returning
this reply code as part of an "unrecognized command"
status places an implementation out of conformance with
this specification.

4.4 Trace information

When an SMTP server receives a message for delivery or further
processing, it MUST insert trace ("time stamp" or "Received")
information at the beginning of the message content, as
discussed under the DATA command in section ##4.1.1.4.

This line must be structured as follows:

* The FROM field SHOULD contain both (1) the name of the
source host as presented in the EHLO or HELO command and (2)
an address literal containing the IP address of the source,
determined from the TCP connection.

* The ID field MAY contain an "@" as suggested in RFC-822,
but this is not required.

* The FOR field MAY contain a list of <path> entries when
multiple RCPT commands have been given.

An Internet mail program MUST NOT change a Received: line that
was previously added to the message header. SMTP servers MUST
prepend Received lines to messages; they MUST NOT change the
order of existing lines or insert Received lines in any other
location.

As the Internet grows, comparability of Received fields is
important for detecting problems, especially slow relays. SMTP
servers that create Received fields SHOULD use explicit offsets
in the dates (e.g., -0800), rather than time zone names of any
type. Local time (with an offset) is preferred to UT when
feasible. If it is desired to also use a time zone name, it
should be included in a commment.

When the delivery SMTP server makes the "final delivery" of a
message, it inserts a return-path line at the beginning of the
mail data. This use of return-path is required; mail systems
MUST support it. The return path line preserves the information
in the <reverse-path> from the MAIL command. Here, final
delivery means the message leaves the SMTP world. Normally,
this would mean it has been delivered to the destination user or
an associated mail drop, but in some cases it may be further
processed and transmitted by another mail system.

It is possible for the mailbox in the return path to be
different from the actual sender's mailbox, for example if error
responses are to be delivered a special error handling mailbox
rather than to that of the message sender. When mailing lists
are involved, this arrangement is common and useful as a means
of directing errors to the list maintainer rather than the
message originator.

The text above implies that the final mail data will begin with
a return path line, followed by one or more time stamp lines.
These lines will be followed by the mail data headers and body
[RFC822]. See Example ##8.

It is sometimes difficult for an SMTP server to determine
whether or not it is making final delivery since forwarding or
other operations may occur after the message is accepted for
delivery. Consequently, any further (forwarding, gateway, or
relay) systems MAY remove the return path and rebuild the MAIL
FROM command as needed to ensure that exactly one such line
appears in a delivered message.

A message-originating SMTP system SHOULD NOT send a message that
already contains a Return-path header. SMTP servers performing a
relay function MUST NOT inspect the message data to the extent needed
to determine if Return-path headers are present. SMTP servers making
final delivery MAY remove Return-path headers before adding their
own.

The primary intent of the Return-path is that it designates the
address to which messages indicating non-delivery or other mail
system failures at to be sent. For this to be unambigious,
exactly one return path should be present when the message is
delivered. Systems using RFC 822 syntax with non-SMTP
transports SHOULD preserve the intent of having an unambiguous
address, associated with the transport envelope, to which to
send error reports (e.g., non-delivery messages).

Historical note: Text in RFC 822 that appears to contradict the
use of Return-path (or the envelope MAIL FROM address) as the
destination of error messages is not applicable on the Internet.
The MAIL FROM address (as copied into the Return-path) MUST be
used as the target of any mail containing delivery error messages.

In particular,

(i) a gateway from SMTP->elsewhere SHOULD insert a return-path
header, unless it is known that the "elsewhere" transport
also uses Internet domain addresses and maintains the
envelope sender address separately.

(ii) a gateway from elsewhere->SMTP SHOULD delete any
return-path header present in the message, and either copy
that information to the SMTP envelope or combine it with
information present in the envelope of the other transport
system to construct the MAIL FROM part of the SMTP envelope.

Special mention is needed of the response and further action required
when the processing following the end of mail data indication is
partially successful. This could arise if after accepting several
recipients and the mail data, the SMTP server finds that the mail
data can be successfully delivered to some of the recipients, but it
cannot be to others (for example, due to mailbox space allocation
problems). In such a situation, the response to the DATA command
must be an OK reply. But, the SMTP server must compose and send an
"undeliverable mail" notification message to the originator of the
message. Either a single notification which lists all of the
recipients that failed to get the message, or separate notification
messages must be sent for each failed recipient (see Example 7). All
undeliverable mail notification messages are sent using the MAIL
command (even if they result from processing the obsolete SEND, SOML,
or SAML commands) and use a null return path as discussed in
section ##3.7.

<<<>>> The following section is incomplete in this draft <<>>

The time stamp line and the return path line are formally defined as
follows:

<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>

<time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>

<from-domain> ::= "FROM" <SP> <domain> <SP>

<by-domain> ::= "BY" <SP> <domain> <SP>

<opt-info> ::= [<via>] [<with>] [<id>] [<for>]

<<>>FOR and <link> need to be nailed down.

<protocol> ::= The standard names for protocols are
registered with the Internet Assigned Numbers Authority
(IANA).

Date ::= Dd SP Mon SP YYYY

Note that the earlier form, which permits two-digit years, has
been deprecated. SMTP systems MUST use four-digit years.

Dd ::= the one or two decimal integer day of the month in
the range 1 to 31.

Mon ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"

YYYY ::= the four decimal integer year in the range 0000 to
9999.

<hh> ::= the two decimal integer hour of the day in the
range 00 to 24.

<mm> ::= the two decimal integer minute of the hour in the
range 00 to 59.

<ss> ::= the two decimal integer second of the minute in the
range 00 to 59.

<zone> ::= A four-digit signed time zone offset, such as -0600 for
US Eastern Standard Time. This may be supplemented by a
time zone name in parentheses, e.g., "-0800 (PDT)". See
##___ for additional discussion.

Note that there is no default; time zone information
is required and MUST be supplied.

-------------------------------------------------------------
|
| Example of Return Path and Received Time Stamps
|
| Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
| Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 -0800
| Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 -0800
| Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 -0800
| Date: 27 Oct 81 15:01:01 -0800 (PST)
| From: JOE@ABC.ARPA
| Subject: Improved Mailing System Installed
| To: SAM@JKL.ARPA
|
| This is to inform you that ...
|
| Example 8
|
-------------------------------------------------------------

4.5. DETAILS

4.5.1. MINIMUM IMPLEMENTATION

In order to make SMTP workable, the following minimum
implementation is required for all receivers:

COMMANDS -- HELO
VRFY
MAIL
RCPT
DATA
RSET
NOOP
QUIT

Any system that includes an SMTP server that supports mail relaying
or delivery, i.e., the RCPT command, MUST support the reserved
mailbox "Postmaster" as a case-insensitive local name. E.g., the
Postmaster address is not strictly necessary if the server always
returns 554 on connection opening as described in section ##3.1. This
requirement to accept mail for Postmaster implies that RCPT TO
commands which specify a mailbox for Postmaster at any of the domains
for which the SMTP server provides mail service as well as the
special case of "RCPT TO:<Postmaster>" (with no domain specification)
MUST be supported.

EHLO SHOULD be supported if possible.

4.5.2. TRANSPARENCY

Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
by the user. In general, users are not aware of such
"forbidden" sequences. To allow all user composed text to be
transmitted transparently the following procedures are used.

1. Before sending a line of mail text the SMTP client checks
the first character of the line. If it is a period, one
additional period is inserted at the beginning of the line.

2. When a line of mail text is received by the SMTP server
it checks the line. If the line is composed of a single
period it is the end of mail. If the first character is a
period and there are other characters on the line, the first
character is deleted.

The mail data may contain any of the 128 ASCII characters. All
characters are to be delivered to the recipient's mailbox
including format effectors and other control characters. If
the transmission channel provides an 8-bit byte (octets) data
stream, the 7-bit ASCII codes are transmitted right justified
in the octets with the high order bits cleared to zero. See
##___ for special treatment of these conditions in SMTP systems
serving a relay function.

In some systems it may be necessary to transform the data as
it is received and stored. This may be necessary for hosts
that use a different character set than ASCII as their local
character set, or that store data in records rather than
strings. If such transforms are necessary, they must be
reversible -- especially if such transforms are applied to
mail being relayed.

4.5.3. SIZES AND TIMEOUTS

There are several objects that have required minimum maximum sizes.
That is, every implementation must be able to receive objects of at
least these sizes, but SHOULD NOT send objects larger than these
sizes. In particular, note that, in some cases, the sizes given may
not be sufficient to accomodate important practices and protocols.
For example, the length specified for a "user name" (local-part) may
not be sufficient to accomodate an encoded X.400 address [RFC-X400].

local-part

The maximum total length of a user name or other local-part
is 64 characters.
<<>>>

domain

The maximum total length of a domain name or number is 255
characters.

path

The maximum total length of a reverse-path or
forward-path is 256 characters (including the punctuation
and element separators).

command line

The maximum total length of a command line including the
command word and the <CRLF> is 512 characters.

reply line

The maximum total length of a reply line including the
reply code and the <CRLF> is 512 characters. More information may
be conveyed through multiple-line replies.

text line

The maximum total length of a text line including the <CRLF> is
1000 characters (but not counting the leading dot duplicated for
transparency). This number may be increased by the use of SMTP
Service Extensions.

message content

The maximum total length of a message content (including any
message headers as well as the message body) MUST BE at least 64K
octets. Especially since the introduction of multimedia mail
[RFC-MIME], message lengths on the Internet have grown
dramatically, and message size restrictions should be avoided
if at all possible. SMTP server systems that must impose
restrictions SHOULD implement the "SIZE" service extension
([RFC-SIZE]) and SMTP client systems that will send large
messages SHOULD utilize it when possible.

recipients buffer

The minimum total number of recipients that must be buffered is
100 recipients. I.e., rejection of messages with fewer than 100
RCPT TO commands is a violation of this specification. The
general principle that relaying SMTP servers MUST NOT, and
delivery SMTP servers SHOULD NOT, perform validation tests on
message headers suggests that rejecting a message based on the
total number of recipients shown in header fields is to be
discouraged. A server which imposes a limit on the number of
recipients MUST behave in an orderly fashion, e.g., reject
additional addresses over its limit rather than silently
discarding addresses previously accepted. A client which needs to
deliver a message that would contain over 100 RCPT TO commands
SHOULD be prepared to transmit it in 100-recipient "chunks" if the
server declines to accept more recipients than that in a single
message.

Errors due to exceeding these limits may be reported by using
the reply codes, for example:

500 Line too long.

501 Path too long

552 Too many recipients.

<<>> Note in draft: Should this be 452?<<>>

552 Too much mail data.

An SMTP client MUST provide a timeout mechanism. It SHOULD use
per-command timeouts rather than somehow trying to time the entire
mail transacton. Timeouts SHOULD be easily reconfigurable,
preferably without recompiling the SMTP code. To implement this, a
timer is set for each SMTP command and for each buffer of the data
transfer. The latter means that the overall timeout is inherently
proportional to the size of the message.

Based on extensive experience with busy mail-relay hosts, the minimum
per-command timeout values SHOULD be as follows:

o Initial 220 Message: 5 minutes

An SMTP client process needs to distinguish between a
failed TCP connection and a delay in receiving the initial
220 greeting message. Many SMTP servers will accept a
TCP connection but delay delivery of the 220 message until
their system load will permit more mail to be processed.

o MAIL Command: 5 minutes

o RCPT Command: 5 minutes

A longer timeout would be required if processing of
mailing lists and aliases were not deferred until after
the message was accepted.

o DATA Initiation: 2 minutes

This is while awaiting the "354 Start Input" reply to a
DATA command.

o Data Block: 3 minutes

This is while awaiting the completion of each TCP SEND
call transmitting a chunk of data.

o DATA Termination: 10 minutes.

This is while awaiting the "250 OK" reply. When the
receiver gets the final period terminating the message
data, it typically performs processing to deliver the
message to a user mailbox. A spurious timeout at this
point would be very wasteful and would typically result in
delivery of multiple copies of the message, since it has been
successfully sent and the server has accepted
responsibility for delivery. See section ##6.1 for
additional discussion.

An SMTP server SHOULD have a timeout of at least 5 minutes
while it is awaiting the next command from the sender.

4.5.4 Queuing Strategies

The common structure of a host SMTP implementation includes user
mailboxes, one or more areas for queueing messages in transit, and
one or more daemon processes for sending and receiving mail. The
exact structure will vary depending on the needs of the users on the
host and the number and size of mailing lists supported by the host.
We describe several optimizations that have proved helpful,
particularly for mailers supporting high traffic levels.

Any queueing strategy MUST include:

o Timeouts on all activities on a per-command basis.

o Never sending error messages in response to error messages.

4.5.4.1 Sending Strategy

The general model of an SMTP client is one or more processes
that periodically attempt to transmit outgoing mail. In a
typical system, the program that composes a message has some
method for requesting immediate attention for a new piece of
outgoing mail, while mail that cannot be transmitted
immediately MUST be queued and periodically retried by the
sender. A mail queue entry will include not only the
message itself but also the envelope information.

The sender MUST delay retrying a particular destination
after one attempt has failed. In general, the retry
interval SHOULD try be at least two addresses.

The following information 30 minutes; however, more
sophisticated and variable strategies will be beneficial
when the SMTP client can determine the reason for non-
delivery.

Retries continue until the message is transmitted or the
sender gives up; the give-up time generally needs to be used at
least 4-5 days. The parameters to rank the host addresses:

(1) Multiple MX Records -- these contain retry algorithm MUST
be configurable.

A client SHOULD keep a preference
indication list of hosts it cannot reach and
corresponding connection timeouts, rather than just retrying queued
mail items.

Experience suggests that should be used in sorting (see below). If there failures are multiple destinations with typically transient (the
target system or its connection has crashed), favoring a policy of
two connection attempts in the same preference and there first hour the message is
no clear reason to favor one (e.g., by address preference), then in the sender-SMTP SHOULD pick one at random
queue, and then backing off to spread once every two or three hours.

The SMTP client can shorten the load
across multiple queueing delay by cooperation with
the SMTP server. In particular, if mail exchangers for is received from a specific organization.

(2) Multihomed host -- The destination
particular address, it is good evidence that any mail queued for
that host (perhaps taken
from the preferred MX record) may can now be multihomed, sent. Application of this principle may, in which
case
many cases, eliminate the domain name resolver will return requirement for an explicit "send queues
now" function such as that discussed in [RFC-ETRN].

The strategy may be further modified as a list of
alternative IP addresses. It is the responsibility result of the
domain name resolver interface multiple
addresses per host (see below), to have ordered this list by
decreasing preference, and optimize delivery time vs.
resource usage.

An SMTP MUST try them client may have a large queue of messages for each
unavailable destination host. If it retried all of these messages
in the order
presented.

Although the capability to try multiple alternative
addresses is required, every retry cycle, there may would be circumstances where
specific installations want to limit or disable excessive Internet overhead and
the use of
alternative addresses. The question of whether daemon would be blocked for a sender
should long period. Note that an SMTP
client can generally determine that a delivery attempt retries using the different addresses has failed
only after a timeout of several minutes; even a
multihomed host has been controversial. The main argument
for using the multiple addresses is that one minute timeout
per connection will result in a very large delay if it maximizes the
probability is repeated
for dozens or even hundreds of timely delivery, and indeed sometimes queued messages to the
probability of any delivery; same host.

At the counter argument is that
it may result in unnecessary resource use.

Note that resource same time, SMTP clients should use great care about caching
negative responses from servers. In an extreme case, if EHLO is also strongly determined
issued multiple times during the same SMTP connection, different
answers may be returned by the
sending strategy discussed in Section #4.5.4.1.

If a host receives a server. More significantly, 5yz
responses to MAIL FROM MUST NOT be cached.

When the same message with a destination for which it is a
designated Mail eXchanger, it MAY make final delivery to be delivered to several users on the same
host, only one copy of the message
or hand it off using some mechanism outside SHOULD be transmitted. That is,
the SMTP-provided
transport environment.

If, instead, it determines that it should relay SMTP client SHOULD use the message, it must
sort command sequence: RCPT, RCPT,... RCPT,
DATA instead of the MX records sequence: RCPT, DATA, ..., RCPT, DATA, ... RCPT,
DATA. Implementation of this efficiency feature is strongly urged.

Similarly, the SMTP client MAY support multiple concurrent outgoing
mail transactions to determine candidates for achieve timely delivery. The records
are first ordered by preference, with However, some limit
may be appropriate to protect the lowest-numbered records
being most preferred. The relay host must then inspect the list for
any of the names or addresses by which it might be known in mail
transactions. If a matching record is found, all records at that
preference level and higher-numbered ones MUST BE discarded from
consideration. If there are no records left at that point, it is an
error condition and the message must be returned as undeliverable.
If records do remain, they should be tried, best preference first, as
described above. devoting all its
resources to mail.

4.5.4.2 Receiving strategy

The theory of MX resolution is discussed in RFC 974, but SMTP server SHOULD attempt to keep a pending listen on the specific
processing rules above supercede it where they are not consistent.

6. Problem detection and handling

6.1 Reliable delivery and replies by email

When SMTP
port at all times. This will require the receiver-SMTP accepts a piece support of mail (by sending a "250 OK"
message in response to DATA), it is accepting responsibility multiple
incoming TCP connections for
delivering or relaying SMTP. Some limit MAY be imposed.

As discussed above, when the message. It must take this responsibility
seriously, i.e., SMTP server receives mail from a
particular host address, it MUST NOT lose could notify the message SMTP client to retry any
mail pending for frivolous reasons,
e.g., because the that host later crashes or because of a predictable
resource shortage.

If there is a delivery failure after acceptance of a message, the
receiver-SMTP MUST formulate address.

5. Address resolution and mail handling

Once an SMTP client lexically identifies a notification message. This
notification MUST domain to which mail is to
be sent using a null ("<>") reverse path delivered for processing (as described in the
envelope. sections ##3.6 and
##3.7), a DNS lookup is performed to resolve that domain name (see
[RFC-DNS]). The recipient of this notification SHOULD be the address
from the envelope return path (or lookup first attempts to locate an MX record
associated with that name. If a CNAME record is found instead, the Return-Path: line). However,
if this address
resulting name is null ("<>"), processed as if it were the receiver-SMTP MUST NOT send a
notification. initial name. If no MX
records are found, but an A RR is found, the address A RR is an explicit source route, treated as if it MUST be
stripped down to its final hop.

DISCUSSION:
For example, suppose that an error notification must be
sent for
was associated with an implicit MX RR, with a message preference of 0,
pointing to that arrived with:
"MAIL FROM:<@a,@b:user@d>". The notification message
should be sent to: "RCPT TO:<user@d>".

Some host.

When the lookup succeeds, the mapping can result in a list of
alternative delivery failures after addresses rather than a single address, because
of (a) multiple MX records, (b) multihoming, or both. To provide
reliable mail transmission, the message is accepted by SMTP will be
unavoidable. For example, it may client MUST be impossible for the receiving
SMTP server able to validate all try (and
retry) each of the delivery relevant addresses in RCPT command(s)
due to this list in order, until a "soft" domain system error, because
delivery attempt succeeds. However, there MAY also be a configurable
limit on the target number of alternate addresses that can be tried. In any
case, a host SHOULD try at least two addresses.

The following information is to be used to rank the host addresses:

(1) Multiple MX Records -- these contain a mailing
list preference
indication that should be used in sorting (see earlier discussion of RCPT), or because below). Lower
numbers are more preferred than higher ones. If there are
multiple destinations with the server is
acting as a relay same preference and has there is
no immediate access clear reason to favor one (e.g., by recognition of an
easily-reached address), then the delivering
system.

To avoid receiving duplicate messages as sender-SMTP SHOULD pick one at
random to spread the result of timeouts, load across multiple mail exchangers for a
receiver-SMTP MUST seek to minimize
specific organization.

(2) Multihomed host -- The destination host (perhaps taken
from the time required to respond to preferred MX record) may be multihomed, in which
case the final "CRLF.CRLF" that ends a message transfer. See RFC-1047
[RFC1047] for domain name resolver will return a discussion of this problem.

6.2 Loop detection

Simple counting list of
alternative IP addresses. It is the number responsibility of Received lines the
domain name resolver interface to have ordered this list by
decreasing preference, and SMTP MUST try them in a message has
proven the order
presented.

Although the capability to try multiple alternative
addresses is required, there may be an effective, although rarely optimal, method of
detecting loops in mail systems. SMTP servers that use this
technique should circumstances where
specific installations want to limit or disable the use a large rejection threshold, normally at
least 100 Received entries. Whatever mechanisms are used,
servers MUST contain provisions for detecting and stopping
trivial loops.

6.3 Compensating for irregularities

Unfortunately, variations, creative interpretations, and outright
violations of Internet mail protocols do occur; some would suggest
that they occur quite frequently.
alternative addresses. The debate as to question of whether a
well-behaved SMTP receiver or relay sender
should reject a malformed
message, attempt to pass it on unchanged, or attempt to repair it to
increase retries using the odds different addresses of successful delivery (or subsequent reply) began
almost with a
multihomed host has been controversial. The main argument
for using the dawn multiple addresses is that it maximizes the
probability of structured network mail timely delivery, and shows no signs of
abating. Advocates indeed sometimes the
probability of rejection claim that attempted repairs are
rarely completely adequate and any delivery; the counterargument is that rejection of bad messages
it may result in unnecessary resource use.

Note that resource use is also strongly determined by the
only way to get
sending strategy discussed in Section #4.5.4.1.

If a host receives a message with a destination for which it is a
designated Mail eXchanger, it MAY relay the offending software repaired. Advocates message, potentially
after having rewritten the addresses, make final delivery of
"repair" the
message, or "deliver no matter what" argue that users prefer that
mail go through hand it if at all possible and that there are significant
market pressures in off using some mechanism outside the
SMTP-provided transport environment.

If it determines that direction. In practice, these market
pressures may be more important to particular vendors then strict
conformance to it should relay the standards, regardless of message without rewriting
the preference of address, it must sort the
actual developers. MX records to determine candidates for
delivery. The problems associated with ill-formed messages were
exacerbated records are first ordered by preference, with the introduction
lowest-numbered records being most preferred. The relay host must
then inspect the list for any of the split-UA mail reading
protocols [RFC-POP2, RFC-POP3, RFC-IMAP2, RFC-PCMAIL]. These
protocols, names or addresses by which facilitate the use of it
might be known in mail transactions. If a client machine,
separate matching record is found,
all records at that preference level and higher-numbered ones MUST BE
discarded from consideration. If there are no records left at that
point, it is an error condition and the one containing the mail-receiving SMTP server,
to actually process incoming mail. These arrangements have
encouraged the use of SMTP message must be returned as
undeliverable. If records do remain, they should be tried, best
preference first, as a posting protocol described above.

6. Problem detection and SMTP
servers as relay systems for these client hosts, which are often
only intermittently connected to the Internet. Historically,
many of those client machines lacked some of the mechanisms handling

6.1 Reliable delivery and
information assumed by SMTP (and indeed, replies by email

When the mail format
protocol [RFC-822]). Some could not keep adequate track of
time; others had no concept of timezones; still others could not
identify their own names or addresses; and, of course, none
could satisfy the assumptions that underlay RFC-822's conception receiver-SMTP accepts a piece of authenticated addresses.

In mail (by sending a "250 OK"
message in response to these weak SMTP clients, many SMTP systems have taken
on the DATA), it is accepting responsibility to complete messages that are delivered to them
in incomplete for
delivering or incorrect form. This strategy is generally
considered to be appropriate when relaying the server can identify or
authenticate message. It must take this responsibility
seriously, i.e., it MUST NOT lose the client and there are prior agreements between them.
By contrast, message for frivolous reasons,
e.g., because the host later crashes or because of a predictable
resource shortage.

If there is at best great concern about fixes applied by a
relay or delivery SMTP server that has little or no knowledge failure after acceptance of a message, the
user or client machine.

The following changes to
receiver-SMTP MUST formulate and mail a message being processed MAY notification message. This
notification MUST be
applied by an originating SMTP server, or one used as the target
of SMTP as an initial posting protocol, when necessary. sent using a null ("<>") reverse path in the
envelope. The
less information recipient of this notification SHOULD be the server has about address
from the client, envelope return path (or the less
likely these changes are to be correct and Return-Path: line). However,
if this address is null ("<>"), the more caution and
conservatism should be applied in considering whether or not to
perform fixes and how. These changes receiver-SMTP MUST NOT be applied by send a
notification. If the address is an
SMTP server explicit source route, it MUST be
stripped down to its final hop.

DISCUSSION:
For example, suppose that provides a simple relay function.

- Addition of a message-id field when none appears

- Addition of an error notification must be
sent for a date, time or timezone when none appears

- Correction of addresses to proper FQDN format

In all cases, properly-operating clients message that supply correct
information are preferred to corrections by the SMTP server. And,
equally in all cases, documentation of actions performed by arrived with:
"MAIL FROM:<@a,@b:user@d>". The notification message
should be sent to: "RCPT TO:<user@d>".

Some delivery failures after the
servers (in trace fields and/or header comments) message is strongly
encouraged.

7. Security Considerations

7.1 Mail security and spoofing accepted by SMTP mail is inherently insecure in that will be
unavoidable. For example, it is feasible may be impossible for even
fairly casual users to negotiate directly with the receiving and
relaying
SMTP servers and create messages that will trick a
naive recipient into believing that they came from somewhere
else. Constructing such server to validate all the delivery addresses in RCPT command(s)
due to a message so that "soft" domain system error, because the "spoofed"
behavior cannot be detected by an expert target is somewhat more
difficult, but not sufficiently so as to be a deterrent to
someone who mailing
list (see earlier discussion of RCPT), or because the server is determined
acting as a relay and knowledgeable.

Consequently, has no immediate access to the delivering
system.

To avoid receiving duplicate messages as knowledge the result of Internet mail increases, so does timeouts, a
receiver-SMTP MUST seek to minimize the
knowledge time required to respond to
the final "CRLF.CRLF" that SMTP mail inherently cannot be authenticated, or
integrity checks provided, at ends a message transfer. See RFC-1047
[RFC1047] for a discussion of this problem.

6.2 Loop detection

Simple counting of the transport level. Real security
lies only number of Received lines in end-to-end methods involving the a message bodies, e.g.,
those that can has
proven to be provided an effective, although rarely optimal, method of
detecting loops in the MOSS framework [RFC-MOSS].

A corollary to this is mail systems. SMTP servers that efforts to make it more difficult use this
technique should use a large rejection threshold, normally at
least 100 Received entries. Whatever mechanisms are used,
servers MUST contain provisions for users to set envelope MAIL FROM detecting and header "From" fields to
point to valid addresses other than their own are largely
misguided: they do not prevent any would-be mail spoofer from
doing so, but they frustrate legitimate applications in which
mail is sent by one user on behalf stopping
trivial loops.

6.3 Compensating for irregularities

Unfortunately, variations, creative interpretations, and outright
violations of another Internet mail protocols do occur; some would suggest
that they occur quite frequently. The debate as to whether a
well-behaved SMTP receiver or in which error
(or normal) replies relay should be directed to reject a special address. On
the other hand, systems that provide convenient ways for users malformed
message, attempt to alter these fields pass it on a per-message basis should unchanged, or attempt to
establish a primary and permanent mailbox address for the user
so that Sender fields within the message data can be generated
sensibly.

This specification does not further address repair it to
increase the authentication
issues associated odds of successful delivery (or subsequent reply) began
almost with SMTP other than to advocate that useful
functionality not be disabled in the hope dawn of providing some
small margin structured network mail and shows no signs of protection against an ignorant user who
abating. Advocates of rejection claim that attempted repairs are
rarely completely adequate and that rejection of bad messages is
trying to fake mail.

7.2 "Blind" copies.

Addresses may appear in the RCPT TO commands
only way to an SMTP server get the offending software repaired. Advocates of
"repair" or "deliver no matter what" argue that do not appear users prefer that
mail go through it if at all possible and that there are significant
market pressures in that direction. In practice, these market
pressures may be more important to particular vendors then strict
conformance to the message headers for a number standards, regardless of
reasons. the preference of the
actual developers.

The two most common problems associated with ill-formed messages were
exacerbated by the introduction of these involve the split-UA mail reading
protocols [RFC-POP2, RFC-POP3, RFC-IMAP2, RFC-PCMAIL]. These
protocols, which facilitate the use of a
mailing address as a "list exploder" -- a single address that
resolves into multiple addresses -- and client machine,
separate from the appearance of "blind
copies". In order to avoid defeating some of one containing the purpose of
these mechanisms, mail-receiving SMTP clients and servers SHOULD NOT copy server,
to actually process incoming mail. These arrangements have
encouraged the
full set use of RCPT TO command arguments into the headers, even SMTP as
informational or private-extension headers. Since this rule is
often violated in practice, a posting protocol and cannot be enforced, sending SMTP
systems that are aware of "bcc" use MAY find it helpful to send
each blind copy
servers as a separate message transaction containing
only a single RCPT TO command.

More generally, while there relay systems for these client hosts, which are often similarities, there is no
inherent relationship between either "reverse" (MAIL FROM, SAML
FROM, etc.) or "forward" (RCPT TO) addresses in the SMTP
transaction ("envelope") and
only intermittently connected to the addresses in Internet. Historically,
many of those client machines lacked some of the headers.
Receiving systems SHOULD NOT attempt to deduce such
relationships mechanisms and use them to alter
information assumed by SMTP (and indeed, by the headers mail format
protocol [RFC-822]). Some could not keep adequate track of the message
for delivery. The popular "Apparently-to" header is a violation
time; others had no concept of this principle and SHOULD NOT be used.

7.3 VRFY, EXPN, and security.

As discussed in section ##3.5, individual sites may want to disable
one timezones; still others could not
identify their own names or both addresses; and, of VRFY or EXPN for security reasons. As a corollary to course, none
could satisfy the above, implementations assumptions that permit this MUST NOT appear underlay RFC-822's conception
of authenticated addresses.

In response to these weak SMTP clients, many SMTP systems have
verified addresses taken
on the responsibility to complete messages that are not, delivered to them
in fact, verified. If a site
disables these commands for security reasons, incomplete or incorrect form. This strategy is generally
considered to be appropriate when the SMTP server MUST
return a 252 response, rather than a code that could be confused with
successful can identify or unsuccessful verification.

Returning a 250 reply code with the address listed in
authenticate the VRFY
command after having checked it only for syntax violates this rule.
Of course, an implementation that "supports" VRFY client and there are prior agreements between them.
By contrast, there is at best great concern about fixes applied by always returning
550 whether a
relay or not the address is valid is equally not in conformance.

Within the last four years, the contents of mailing lists have become
popular as an address information source for so-called "spammers".
The use of EXPN to "harvest" addresses delivery SMTP server that has increased as the
administrators of the lists have devised and installed protections
against inappropriate uses little or no knowledge of the lists themselves. Implementations
SHOULD still provide support for EXPN, but sites should carefully
evaluate the tradeoffs between the advantages of having EXPN
available for debugging purposes and
user or client machine.

The following changes to a message being processed MAY be
applied by an originating SMTP server, or one used as the disadvantages target
of exposing SMTP as an initial posting protocol, when necessary. The
less information that may be used in ways that list members may consider
inappropriate. As authentication mechanisms are introduced into
SMTP, some sites may choose to make EXPN available only to
authenticated requestors.

7.4. Information Disclosure

There the server has been an ongoing debate about the tradeoffs between client, the
debugging advantages of announcing server type less
likely these changes are to be correct and version (and,
sometimes, even server domain name) in the greeting response or more caution and
conservatism should be applied in response considering whether or not to the HELP command
perform fixes and exposing useful information
to potential hostile attack. The utility of the debugging
information is beyond doubt. Those who argue for making it
available point out that it is far better to actually secure how. These changes MUST NOT be applied by an
SMTP server than to hope that trying provides a simple relay function.

- Addition of a message-id field when none appears

- Addition of a date, time or timezone when none appears

- Correction of addresses to conceal known
vunerabilities by hiding the server's precise identity will
provide more than momentary protection. Sites proper FQDN format

In all cases, properly-operating clients that supply correct
information are encoruaged preferred to
evaluate corrections by the tradeoff with that issue SMTP server. And,
equally in mind; implementations
are all cases, documentation of actions performed by the
servers (in trace fields and/or header comments) is strongly encouraged to at least provide for making type
encouraged.

7. Security Considerations

7.1 Mail security and
version information available spoofing

SMTP mail is inherently insecure in some way that it is feasible for even
fairly casual users to other network
hosts.

7.5. Scope of operation of negotiate directly with receiving and
relaying SMTP servers

It is and create messages that will trick a well-established principle
naive recipient into believing that an SMTP server may
refuse to accept mail for any operational or technical reason they came from somewhere
else. Constructing such a message so that makes sense to the site providing the server. This is, of
course, just "spoofed"
behavior cannot be detected by an example expert is somewhat more
difficult, but not sufficiently so as to be a deterrent to
someone who is determined and knowledgeable.

Consequently, as knowledge of Internet mail increases, so does the principle
knowledge that sites on the
Internet should not SMTP mail inherently cannot be able to compel other sites to incur costs
on their behalf. On authenticated, or
integrity checks provided, at the other hand, transport level. Real security
lies only in end-to-end methods involving the Internet operates, message bodies, e.g.,
those that can be provided in
many respects, by cooperation among sites and installations.
Obviously, if sites take excessive advantage of the right MOSS framework [RFC-MOSS].

A corollary to
reject traffic, the ubiquity of email availability this is that has been efforts to make it more difficult
for users to set envelope MAIL FROM and header "From" fields to
point to valid addresses other than their own are largely
misguided: they do not prevent any would-be mail spoofer from
doing so, but they frustrate legitimate applications in which
mail is sent by one user on behalf of the strengths of the Internet will be threatened;
considerable care another or in which error
(or normal) replies should be taken and balance maintained if a
site decides directed to be selective about a special address. On
the traffic other hand, systems that is accepted provide convenient ways for users
to alter these fields on a per-message basis should attempt to
establish a primary and processed.

In recent years, use of permanent mailbox address for the relay function through arbitrary
sites has sometimes been used as part of hostile efforts to hide user
so that Sender fields within the actual origins of mail, either to initiate unsolicited (and
difficult-to-trace) mass mailings or to mount denial of service
or message data can be generated
sensibly.

This specification does not further address the authentication
issues associated with SMTP other attacks. Some sites have consequently decided than to
limit advocate that useful
functionality not be disabled in the use hope of the relay function to known or identifiable
sources and implementations SHOULD provide the capability to
perform this type providing some
small margin of filtering. When mail protection against an ignorant user who is rejected for these
or other policy reasons, a 550 code should be used in response
trying to EHLO, MAIL FROM, or fake mail.

7.2 "Blind" copies.

Addresses may appear in the RCPT TO as appropriate.

8. IANA Considerations

IANA is [[requested]] commands to set up two registries. The first consists
of SMTP service extensions with the associated keywords, and, as
needed, parameters and verbs. As specified in section ##2.2.2, no
entry may be made in this registry that starts in an "X". Entries
may be made only for service extensions (and associated keywords,
parameters, or verbs) SMTP server
that are defined do not appear in standards-track or
experimental RFCs specifically approved by IESG the message headers for this purpose. a number of
reasons. The second registry consists two most common of "tags" that identify forms these involve the use of domain
literals other than those for IPv4 a
mailing address as a "list exploder" -- a single address that
resolves into multiple addresses (specified in RFC 821
and in this document) -- and IPv6 addresses (specified in this
document). Additional literal types require standardization before
being used; none of these are anticipated at this time.

9. REFERENCES

[1] ASCII

ASCII, "USA Code for Information Interchange", United States of
America Standards Institute (now American National Standards
Institute), X3.4, 1968. ANSI X3.4-1968 has been replaced by
newer versions with slight modifications, but the 1968
version remains definitive for the Internet.

[RFC822]
Crocker, D., "Standard for the Format appearance of ARPA Internet Text
Messages", RFC 822, Department "blind
copies". In order to avoid defeating some of Electrical Engineering,
University the purpose of Delaware, August 1982.

[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981. Also in:
Feinler, E. and J. Postel, eds., "Internet Protocol Transition
Workbook", SRI International, Menlo Park, California, March 1982.

[HEADER-PEOPLE]

[RFC-DNS] P. Mockapetris, "Domain names - implementation and
specification", RFC 1035 and P. Mockapetris, "Domain names -
concepts and facilities", RFC 1034. (STD 13)

[RFC974] C. Partridge, "Mail routing
these mechanisms, SMTP clients and servers SHOULD NOT copy the domain system", RFC
974, 01/01/1986

[RFC1047] C. Partridge, "Duplicate messages and SMTP", RFC 1047,
02/01/1988.

[RFC-SIZE] J. Klensin, N. Freed, K. Moore, "SMTP Service Extension
for Message Size Declaration", RFC 1870, 11/06/1995. (STD 10)

[8BITMIME] J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker,
"SMTP Service Extension for 8bit-MIMEtransport", RFC 1652,
07/18/1994.

[SMTPEX] J. Klensin, N. Freed, M. Rose, E. Stefferud, D.
Crocker, "SMTP Service Extensions", RFC-1869, 11/06/1995.

[RFC-1123] R. Braden, "Requirements for Internet hosts -
application
full set of RCPT TO command arguments into the headers, even as
informational or private-extension headers. Since this rule is
often violated in practice, and support", 10/01/1989

[RFC-MOSS] S. Crocker, N. Freed, J. Galvin, S. Murphy, "MIME Object
Security Services", RFC 1848, 10/03/1995.

[RFC-POP2] M. Butler, D. Chase, J. Goldberger, J. Postel, J.
Reynolds, "Post Office Protocol - version 2", RFC 937,
02/01/1985

[RFC-IMAP2] M. Crispin, "Interactive Mail Access Protocol - Version
2", RFC 1176, 08/20/1990.

[RFC-PCMAIL] M. Lambert, "PCMAIL: A distributed mail system for
personal computers", RFC 1056, 06/01/1988.

[RFC-POP3] J. Myers, M. Rose, "Post Office Protocol - Version 3",
RFC 1930, 5/14/96 (Std 53).

[RFC-IMAP4] M. Crispin, "Internet Message Access Protocol
- Version 4", RFC 2060, 12/04/1996.

[RFC-X400] S. Hardcastle-Kille, "Mapping cannot be enforced, sending SMTP
systems that are aware of "bcc" use MAY find it helpful to send
each blind copy as a separate message transaction containing
only a single RCPT TO command.

More generally, while there are often similarities, there is no
inherent relationship between X.400(1988) /
ISO 10021 either "reverse" (MAIL FROM, SAML
FROM, etc.) or "forward" (RCPT TO) addresses in the SMTP
transaction ("envelope") and RFC 822", RFC 1327, 05/18/1992.

[RFC-ETRN] J. De Winter, "SMTP Service Extension for Remote
Message Queue Starting", RFC 1985, 08/14/1996.

[RFC-BDAT] G. Vaudreuil, "SMTP Service Extensions the addresses in the headers.
Receiving systems SHOULD NOT attempt to deduce such
relationships and use them to alter the headers of the message
for
Transmission delivery. The popular "Apparently-to" header is a violation
of Large this principle and Binary MIME Messages", RFC 1830,
08/16/1995.

[RFC-PIPELINE] N. Freed, A. Cargille, "SMTP Service Extension
for Command Pipelining", RFC 1854, 10/04/1995.

[RFC-NOTARY1] K. Moore, "SMTP Service Extension for Delivery
Status Notifications", RFC 1891, 01/15/1996.

[RFC-NOTARY2] K. Moore, G. Vaudreuil, "An Extensible Message
Format for Delivery Status Notifications", RFC 1894,
01/15/1996.

[RFC-REPLY] G. Vaudreuil, "Enhanced Mail System Status Codes",
RFC 1893, 01/15/1996.

[ABNF] Crocker, D., "Augmented BNF SHOULD NOT be used.

7.3 VRFY, EXPN, and security.

As discussed in section ##3.5, individual sites may want to disable
one or both of VRFY or EXPN for Syntax Specifications: ABNF",
(in progress -- draft-ietf-drums-abnf-01.txt)

[MSGFMT] P. Resnick, Work security reasons. As a corollary to
the above, implementations that permit this MUST NOT appear to have
verified addresses that are not, in progress,
draft-ietf-drums-msg-fmt-01.txt, March 1997.

9. Editor's Addresses

John C. Klensin
MCI Communications
800 Boylston St, 7th floor
Boston, MA 02199
USA
Email: Klensin@mci.net
Phone: +1 617 960 1011
Fax: +1 617 960 1009

10. Acknowledgements

<<to fact, verified. If a site
disables these commands for security reasons, the SMTP server MUST
return a 252 response, rather than a code that could be supplied>>

APPENDIX A

TCP Transport service

The Transmission Control Protocol [3] is used in confused with
successful or unsuccessful verification.

Returning a 250 reply code with the Internet, and address listed in
any network following the Internet standards VRFY
command after having checked it only for internetwork protocols.

Connection Establishment

The SMTP transmission channel is a TCP connection established
between the sender process port U and syntax violates this rule.
Of course, an implementation that "supports" VRFY by always returning
550 whether or not the receiver process port
L. This single full duplex connection address is used as the
transmission channel. This protocol valid is assigned equally not in conformance.

Within the service
port 25 (31 octal), that is L=25.

Data Transfer

The TCP connection supports last four years, the transmission contents of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted mailing lists have become
popular as an 8-bit byte with the high-order bit cleared to
zero.

APPENDIX B

Generating SMTP commands from RFC 822 headers

Some systems address information source for so-called "spammers".
The use RFC 822 headers (only) in a mail submission
protocol, or otherwise generate SMTP commands from RFC 822 headers
when such a message is handed of EXPN to an MTA from a UA. While "harvest" addresses has increased as the MTA-UA
protocol is a private matter, not covered by any Internet Standard,
there are problems with this approach. For example, there
administrators of the lists have been
repeated problems with proper handling devised and installed protections
against inappropriate uses of the lists themselves. Implementations
SHOULD still provide support for EXPN, but sites should carefully
evaluate the tradeoffs between the advantages of "bcc" copies having EXPN
available for debugging purposes and
redistribution lists when the disadvantages of exposing
information that conceptually belongs to a
mail envelopes is not separated early may be used in processing from header
information (and kept separate).

It is recommended ways that the UA provide its initial MTA with list members may consider
inappropriate. As authentication mechanisms are introduced into
SMTP, some sites may choose to make EXPN available only to
authenticated requestors.

7.4. Information Disclosure

There has been an
envelope separate from ongoing debate about the message itself. However, if tradeoffs between the envelope
is not supplied, SMTP commands should be generated as follows:

(i) each recipient address from a TO, CC,
debugging advantages of announcing server type and version (and,
sometimes, even server domain name) in the greeting response or BCC header field
should be copied
in response to a RCPT the HELP command (generating multiple message
copies if that and exposing useful information
to potential hostile attack. The utility of the debugging
information is required beyond doubt. Those who argue for queuing or delivery). This includes
any addresses listed in a RFC 822 "group". Any BCC fields should
then be removed from the headers. Once this process making it
available point out that it is completed, far better to actually secure an
SMTP server than to hope that trying to conceal known
vunerabilities by hiding the remaining headers should be checked server's precise identity will
provide more than momentary protection. Sites are encoruaged to
evaluate the tradeoff with that issue in mind; implementations
are strongly encouraged to verify that at least one
To:, Cc:, or Bcc: header remains. If none do, then a bcc: header
with no additional provide for making type and
version information SHOULD be inserted (see section
##2.15 <<>>??).

(ii) the return address available in some way to other network
hosts.

7.5. Scope of operation of SMTP servers

It is a well-established principle that an SMTP server may
refuse to accept mail for any operational or technical reason
that makes sense to the MAIL command should be derived from site providing the system's identity for server. This is, of
course, just an example of the submitting (local) user. That return
address principle that sites on the
Internet should also not be copied able to compel other sites to incur costs
on their behalf. On the Sender header field if it is
different from other hand, the address Internet operates, in
many respects, by cooperation among sites and installations.
Obviously, if sites take excessive advantage of the From header field. (Any Sender
field right to
reject traffic, the ubiquity of email availability that was already there has been
one of the strengths of the Internet will be threatened;
considerable care should be removed.) Systems may provide taken and balance maintained if a way for submitters
site decides to override be selective about the envelope return address, but may
want traffic that is accepted
and processed.

In recent years, use of the relay function through arbitrary
sites has sometimes been used as part of hostile efforts to hide
the actual origins of mail, either to initiate unsolicited (and
difficult-to-trace) mass mailings or to mount denial of service
or other attacks. Some sites have consequently decided to restrict its
limit the use of the relay function to privileged users. (This will not prevent known or identifiable
sources and implementations SHOULD provide the capability to
perform this type of filtering. When mail forgery, but may lessen its incidence -- see section 7.1.)

A submission protocol based on Standard RFC 822 information alone
MUST NOT is rejected for these
or other policy reasons, a 550 code should be used in response
to gateway a message from a foreign (non-SMTP) mail
system into an SMTP environment. Additional information EHLO, MAIL FROM, or RCPT TO as appropriate.

8. IANA Considerations

IANA is [[requested]] to construct set up two registries. The first consists
of SMTP service extensions with the associated keywords, and, as
needed, parameters and verbs. As specified in section ##2.2.2, no
entry may be made in this registry that starts in an envelope must come from some source "X". Entries
may be made only for service extensions (and associated keywords,
parameters, or verbs) that are defined in standards-track or
experimental RFCs specifically approved by IESG for this purpose.

The second registry consists of "tags" that identify forms of domain
literals other than those for IPv4 addresses (specified in RFC 821
and in this document) and IPv6 addresses (specified in this
document). Additional literal types require standardization before
being used; none of these are anticipated at this time.

9. REFERENCES

[1] ASCII

Attempts to gateway messages using only their header "to" and "cc"
fields, have repeatedly caused mail loops and other behavior adverse
to Internet.

[RFC822]
Crocker, D., "Standard for the proper functioning Format of the ARPA Internet mail environment. These
problems have been especially common when the message originates from
an Text
Messages", RFC 822, Department of Electrical Engineering,
University of Delaware, August 1982.

[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet mailing list
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981. Also in:
Feinler, E. and J. Postel, eds., "Internet Protocol Transition
Workbook", SRI International, Menlo Park, California, March 1982.

[HEADER-PEOPLE]

[RFC-DNS] P. Mockapetris, "Domain names - implementation and
specification", RFC 1035 and P. Mockapetris, "Domain names -
concepts and facilities", RFC 1034. (STD 13)

[RFC974] C. Partridge, "Mail routing and is distributed into the foreign
environment using envelope information. When these domain system", RFC
974, 01/01/1986

[RFC1047] C. Partridge, "Duplicate messages are then
processed by a header-only remailer, loops back to the Internet
environment (and the mailing list) are almost inevitable.

APPENDIX C

Source routes.

The <reverse-path> is a reverse source routing
list of and SMTP", RFC 1047,
02/01/1988.

[RFC-SIZE] J. Klensin, N. Freed, K. Moore, "SMTP Service Extension
for Message Size Declaration", RFC 1870, 11/06/1995. (STD 10)

[8BITMIME] J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker,
"SMTP Service Extension for 8bit-MIMEtransport", RFC 1652,
07/18/1994.

[SMTPEX] J. Klensin, N. Freed, M. Rose, E. Stefferud, D.
Crocker, "SMTP Service Extensions", RFC-1869, 11/06/1995.

[RFC-1123] R. Braden, "Requirements for Internet hosts -
application and source mailbox. The first host in the
<reverse-path> should be the host sending this command.

However, in general, the <forward-path>
should contain only a mailbox and domain name, relying on the
domain name support", 10/01/1989

[RFC-MOSS] S. Crocker, N. Freed, J. Galvin, S. Murphy, "MIME Object
Security Services", RFC 1848, 10/03/1995.

[RFC-POP2] M. Butler, D. Chase, J. Goldberger, J. Postel, J.
Reynolds, "Post Office Protocol - version 2", RFC 937,
02/01/1985

[RFC-IMAP2] M. Crispin, "Interactive Mail Access Protocol - Version
2", RFC 1176, 08/20/1990.

[RFC-PCMAIL] M. Lambert, "PCMAIL: A distributed mail system to supply routing information if required.

For relay purposes, the forward-path may be a source route of the form
"@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST BE
fully-qualified domain names. This form is used to emphasize the
distinction for
personal computers", RFC 1056, 06/01/1988.

[RFC-POP3] J. Myers, M. Rose, "Post Office Protocol - Version 3",
RFC 1930, 5/14/96 (Std 53).

[RFC-IMAP4] M. Crispin, "Internet Message Access Protocol
- Version 4", RFC 2060, 12/04/1996.

[RFC-X400] S. Hardcastle-Kille, "Mapping between an address X.400(1988) /
ISO 10021 and a route. The mailbox is an
absolute address, RFC 822", RFC 1327, 05/18/1992.

[RFC-ETRN] J. De Winter, "SMTP Service Extension for Remote
Message Queue Starting", RFC 1985, 08/14/1996.

[RFC-BDAT] G. Vaudreuil, "SMTP Service Extensions for
Transmission of Large and the route is information about how to get
there. The two concepts should not be confused.

If source routes are
used, Binary MIME Messages", RFC 1830,
08/16/1995.

[RFC-PIPELINE] N. Freed, A. Cargille, "SMTP Service Extension
for Command Pipelining", RFC 821 should be consulted 1854, 10/04/1995.

[RFC-NOTARY1] K. Moore, "SMTP Service Extension for the mechanisms Delivery
Status Notifications", RFC 1891, 01/15/1996.

[RFC-NOTARY2] K. Moore, G. Vaudreuil, "An Extensible Message
Format for constructing
and updating the forward- and reverse-paths.

The SMTP server transforms
the command arguments by moving its own identifier (its domain name
or that of any domain Delivery Status Notifications", RFC 1894,
01/15/1996.

[RFC-REPLY] G. Vaudreuil, "Enhanced Mail System Status Codes",
RFC 1893, 01/15/1996.

[ABNF] Crocker, D., "Augmented BNF for which it Syntax Specifications: ABNF",
(in progress -- draft-ietf-drums-abnf-01.txt)

[MSGFMT] P. Resnick, Work in progress,
draft-ietf-drums-msg-fmt-01.txt, March 1997.

9. Editor's Addresses

John C. Klensin
MCI Communications
800 Boylston St, 7th floor
Boston, MA 02199
USA
Email: Klensin@mci.net
Phone: +1 617 960 1011
Fax: +1 617 960 1009

10. Acknowledgements

<<>>to be supplied>>

APPENDIX A

TCP Transport service

The Transmission Control Protocol [3] is acting as a mail exchanger), if
it appears, from the forward-path to the beginning of the
reverse-path.

Notice that the forward-path and reverse-path appear used in the SMTP
commands Internet, and replies, but not necessarily in
any network following the message. That is,
there is no need Internet standards for these paths and especially this syntax to appear
in the "To:" , "From:", "CC:", etc. fields of the message header.
Conversely, SMTP servers MUST NOT derive final message delivery
information from message header fields.

When the list of hosts is present, it is a "reverse" source
route and indicates that the mail was relayed through each host on
the list (the first host in internetwork protocols.

Connection Establishment

The SMTP transmission channel is a TCP connection established
between the list was sender process port U and the most recent relay). receiver process port
L. This list single full duplex connection is used as a source route to return non-delivery notices to
the sender. As each relay host adds itself to the beginning of the
list, it must use its name as known in the transport environment to
which it
transmission channel. This protocol is relaying assigned the mail rather than service
port 25 (31 octal), that of the transport
environment from which the mail came (if they are different).

APPENDIX E

Theory of Reply Codes is L=25.

Data Transfer

The three digits of TCP connection supports the reply each have a special significance. transmission of 8-bit bytes.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated SMTP client, or one that
receives data is 7-bit ASCII characters. Each character is
transmitted as an unexpected code, will be able 8-bit byte with the high-order bit cleared to determine its next
action (proceed as planned, redo, retrench, etc.) by simply
examining
zero. Service extensions may modify this first digit. An SMTP client that wants rule to know
approximately what kind of error occurred (e.g., mail system
error, command syntax error) may examine the second digit,
reserving the third digit and any supplemental information that
may be present for the finest gradation of information.

There are five values for the first digit of the reply code:

1yz Positive Preliminary reply

The command has been accepted, but the requested action
is being held in abeyance, pending confirmation permit
transmission of full 8-bit data bytes as part of the
information message
body, but not in this reply. The SMTP client should send
another command specifying whether to continue commands or abort
the action.

[Note: responses.

APPENDIX B

Generating SMTP does commands from RFC 822 headers

Some systems use RFC 822 headers (only) in a mail submission
protocol, or otherwise generate SMTP commands from RFC 822 headers
when such a message is handed to an MTA from a UA. While the MTA-UA
protocol is a private matter, not have covered by any commands that allow Internet Standard,
there are problems with this
type approach. For example, there have been
repeated problems with proper handling of reply, "bcc" copies and so does
redistribution lists when information that conceptually belongs to a
mail envelopes is not have separated early in processing from header
information (and kept separate).

It is recommended that the continue or
abort commands.]

2yz Positive Completion reply

The requested action has been successfully completed. A
new request may be initiated.

3yz Positive Intermediate reply

The command has been accepted, but UA provide its initial MTA with an
envelope separate from the requested action message itself. However, if the envelope
is being held in abeyance, pending receipt of further
information. The not supplied, SMTP client commands should send another be generated as follows:

(i) each recipient address from a TO, CC, or BCC header field
should be copied to a RCPT command
specifying this information. This reply (generating multiple message
copies if that is used in
command sequence groups.

4yz Transient Negative Completion reply

The command was not accepted and the requested action did
not occur. However, required for queuing or delivery). This includes
any addresses listed in a RFC 822 "group". Any BCC fields should
then be removed from the error condition headers. Once this process is temporary and completed,
the action may be requested again. The sender remaining headers should
return be checked to verify that at least one
To:, Cc:, or Bcc: header remains. If none do, then a bcc: header
with no additional information SHOULD be inserted as specified in
[MSGFMT].

(ii) the beginning of return address in the MAIL command sequence (if any).
It should, if possible, be
derived from the system's identity for the submitting (local) user.
And the From header field otherwise. If there is difficult to assign a meaning system identity
available, it should also be copied to "transient" when
two the Sender header field if it
is different sites (receiver- and sender- SMTPs) must
agree on from the interpretation. Each reply address in this category
might have the From header field. (Any Sender
field that was already there should be removed.) Systems may provide
a different time value, but way for submitters to override the SMTP client is
encouraged envelope return address, but may
want to try again. A rule of thumb restrict its use to determine if
a reply fits into the 4yz or the 5yz category (see below) privileged users. (This will not prevent
mail forgery, but may lessen its incidence -- see section 7.1.)

When an MTA is that replies are 4yz if they can be repeated without
any change in command form or being used in properties of the sender
or receiver. (E.g., the command is repeated identically
and this way, it bears responsibility for
ensuring that the receiver does not put up a new implementation.)

5yz Permanent Negative Completion reply message being transmitted is valid. The command was not accepted mechanisms
for checking that validity, and the requested action did for handling (or returning) messages
that are not occur. The SMTP client is discouraged from repeating valid at the exact request (in time of arrival, are part of the same sequence). Even some
"permanent" error conditions can MUA-MTA
interface and not covered by this specification.

A submission protocol based on Standard RFC 822 information alone
MUST NOT be corrected, so the
human user may want used to direct the gateway a message from a foreign (non-SMTP) mail
system into an SMTP client environment. Additional information to
reinitiate the command sequence by direct action at construct
an envelope must come from some
point source in the future (e.g., after the spelling has been
changed, other environment,
whether supplemental headers or the user has altered the account status).

The second digit encodes responses in specific categories:

x0z Syntax -- These replies refer foreign system's envelope.

Attempts to syntax errors,
syntactically correct commands that don't fit any
functional category, gateway messages using only their header "to" and unimplemented or superfluous
commands.

x1z Information -- These are replies "cc"
fields, have repeatedly caused mail loops and other behavior adverse
to requests for
information, such as status or help.

x2z Connections -- the proper functioning of the Internet mail environment. These
problems have been especially common when the message originates from
an Internet mailing list and is distributed into the foreign
environment using envelope information. When these messages are replies referring then
processed by a header-only remailer, loops back to the
transmission channel.

x3z Unspecified as yet.

x4z Unspecified as yet.

x5z Mail system -- These replies indicate Internet
environment (and the status mailing list) are almost inevitable.

APPENDIX C

Source routes.

The <reverse-path> is a reverse source routing list of hosts and
source mailbox. The first host in the receiver mail system vis-a-vis <reverse-path> should be the requested
transfer or other mail
host sending this command.

However, in general, the <forward-path> should contain only a
mailbox and domain name, relying on the domain name system action.

The third digit gives to
supply routing information if required.

For relay purposes, the forward-path may be a finer gradation source route of meaning in each
category specified by the second digit. The list of replies
illustrates this. Each reply text is recommended rather than
mandatory,
form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and may even change according THREE MUST BE
fully-qualified domain names. This form is used to emphasize the command with
which it
distinction between an address and a route. The mailbox is associated. On the other hand, the reply codes
must strictly follow an
absolute address, and the specifications in this section.
Receiver implementations route is information about how to get
there. The two concepts should not invent new codes be confused.

If source routes are used, RFC 821 should be consulted for
slightly different situations from the ones described here, but
rather adapt codes already defined.

For example, a command such as NOOP whose successful execution
does not offer
mechanisms for constructing and updating the forward- and
reverse-paths.

The SMTP client server transforms the command arguments by moving its own
identifier (its domain name or that of any new information will return
a 250 reply. The response domain for which it is 502 when
acting as a mail exchanger), if it appears, from the command requests an
unimplemented non-site-specific action. A refinement forward-path to
the beginning of that
is the 504 reply for a command reverse-path.

Notice that is implemented, the forward-path and reverse-path appear in the SMTP
commands and replies, but that
requests an unimplemented parameter.

The reply text may be longer than a single line; not necessarily in the message. That is,
there is no need for these cases paths and especially this syntax to appear
in the complete text must be marked so "To:" , "From:", "CC:", etc. fields of the message header.
Conversely, SMTP client knows when it
can stop reading servers MUST NOT derive final message delivery
information from message header fields.

When the reply. This requires a special format to
indicate list of hosts is present, it is a multiple line reply.

The format for multiline replies requires "reverse" source route
and indicates that every line,
except the last, begin with mail was relayed through each host on the reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with list
(the first host in the reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.

For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line

In many cases list was the SMTP client then simply needs most recent relay). This list is
used as a source route to return non-delivery notices to search for the reply code followed by <SP> at sender.
As each relay host adds itself to the beginning of a line, and
ignore all preceding lines. In a few cases, there is important
data for the sender list, it must
use its name as known in the reply "text". The sender will know
these cases transport environment to which it is
relaying the mail rather than that of the transport environment from
which the current context. mail came (if they are different).

APPENDIX F

Scenarios

This section presents complete scenarios of several types of SMTP
sessions.

A Typical SMTP Transaction Scenario

This SMTP example shows mail sent by Smith at host USC-ISIF, to
Jones, Green, and Brown at host BBN-UNIX. Here we assume that host
USC-ISIF contacts host BBN-UNIX directly. The mail is accepted for
Jones and Brown. Green does not have a mailbox at host BBN-UNIX.

-------------------------------------------------------------

R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA

S: MAIL FROM:<Smith@USC-ISIF.ARPA>
R: 250 OK

S: RCPT TO:<Jones@BBN-UNIX.ARPA>
R: 250 OK

S: RCPT TO:<Green@BBN-UNIX.ARPA>
R: 550 No such user here

S: RCPT TO:<Brown@BBN-UNIX.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK

S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel

Scenario 1

-------------------------------------------------------------

Aborted SMTP Transaction Scenario

-------------------------------------------------------------

R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
S: HELO ISI-VAXA.ARPA
R: 250 MIT-Multics.ARPA

S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
R: 250 OK

S: RCPT TO:<Jones@MIT-Multics.ARPA>
R: 250 OK

S: RCPT TO:<Green@MIT-Multics.ARPA>
R: 550 No such user here

S: RSET
R: 250 OK

S: QUIT
R: 221 MIT-Multics.ARPA Service closing transmission channel

Scenario 2

-------------------------------------------------------------

Relayed Mail Scenario

-------------------------------------------------------------

Step 1 -- Source Host to Relay Host

R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-AI.ARPA
R: 250 USC-ISIE.ARPA

S: MAIL FROM:<JQP@MIT-AI.ARPA>
R: 250 OK

S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK

S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel

Step 2 -- Relay Host to Destination Host

R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIE.ARPA
R: 250 BBN-VAX.ARPA

S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
R: 250 OK

S: RCPT TO:<Jones@BBN-VAX.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
2 Nov 81 22:40:10 UT
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK

S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel

Scenario 3

-------------------------------------------------------------

Verifying and Sending Scenario

-------------------------------------------------------------

R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA

S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

S: SEND FROM:<EAK@MIT-MC.ARPA>
R: 250 OK

S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK

S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel

Scenario 4

-------------------------------------------------------------

Mailing List Scenario

First each of two mailing lists are expanded in separate sessions
with different hosts. Then the message is sent to everyone that
appeared on either list (but no duplicates) via a relay host.

-------------------------------------------------------------

Step 1 -- Expanding the First List

R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-AI.ARPA

S: EXPN Example-People
R: 250-<ABC@MIT-MC.ARPA>
R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<joe@foo-unix.ARPA>
R: 250 <xyz@bar-unix.ARPA>

S: QUIT
R: 221 MIT-AI.ARPA Service closing transmission channel

Step 2 -- Expanding the Second List

R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-MC.ARPA

S: EXPN Interested-Parties
R: 250-Al Calico <ABC@MIT-MC.ARPA>
R: 250-<XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<fred@BBN-UNIX.ARPA>
R: 250 <xyz@bar-unix.ARPA>

S: QUIT
R: 221 MIT-MC.ARPA Service closing transmission channel

Step 3 -- Mailing to All via a Relay Host

R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 USC-ISIE.ARPA

S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
R: 250 OK
S: RCPT
TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK

S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel

Scenario 7

-------------------------------------------------------------

Too Many Recipients Scenario

-------------------------------------------------------------

R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BERKELEY.ARPA

S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK

S: RCPT TO:<fabry@BERKELEY.ARPA>
R: 250 OK

S: RCPT TO:<eric@BERKELEY.ARPA>
R: 552 Recipient storage full, try again in another transaction

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK

S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK

S: RCPT TO:<eric@BERKELEY.ARPA>
R: 250 OK

S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK

S: QUIT
R: 221 BERKELEY.ARPA Service closing transmission channel

Scenario 10

-------------------------------------------------------------

Note that a real implementation must handle many recipients as
specified in Section ##4.5.3.

APPENDIX G Other gateway issues.

In general, gateways between the Internet and other mail systems
SHOULD attempt to preserve any layering semantics across the
boundaries between the two mail systems involved. Gateway-
translation approaches that attempt to take shortcuts by mapping,
e.g., envelope information from one system to the message headers or
body of another have generally proven to be inadequate in important
ways. Systems translating between environments that do not support
both envelopes and headers and Internet mail must be written with the
understanding that some information loss is almost inevitable.

APPENDIX I: Deprecated features of RFC 821

A few features of RFC 821 have proven to be problematic and should
not be used in Internet mail. These are:

(1) TURN

This command, described in RFC 821, raises important security issues
(described in RFC 1123). Its use is deprecated; SMTP systems SHOULD
NOT use it unless the server can authenticate the client.

(2) Source routing

RFC 821 utilized the concept of explicit source routing to get mail
from one host to another via a series of relays. The requirement to
utilize source routes in regular mail traffic was eliminated by the
introduction of the domain name system "MX" record and the last
significant justification for them was eliminated by the
introduction, in RFC 1123, of a clear requirement that addresses
following an "@" must all be fully-qualified domain names.
Consequently, the only remaining justifications for the use of source
routes are support for very old SMTP clients or MUAs and in mail
system debugging. They can, however, still be useful in the latter
circumstance and for routing mail around serious, but temporary,
problems such as problems with the relevant DNS records.

SMTP servers MUST continue to accept source route syntax as specified
in the main body of this document and in RFC 1123. They MAY, if
necessary, ignore the routes and utilize only the target domain in
the address. If they do utilize the source route, the message MUST
be sent to the first domain shown in the address. In particular, a
server MUST NOT guess at shortcuts within the source route.

Clients SHOULD NOT utilize explicit source routing except under
unusual circumstances, such as debugging or potentially relaying
around firewall or mail system configuration errors.

(3) HELO

As discussed in sections ##3.1 and ##4.1.1, EHLO is strongly
preferred to HELO when the server will accept the former. Servers
must continue to accept HELO in order to support older clients.

(4) #-literals

RFC 821 provided for specifying an Internet address as a decimal
integer host number prefixed by a pound sign, "#". In practice, that
form has been obsolete since the introduction of TCP/IP. It is
deprecated and MUST NOT be used.

(5) Dates and years

When dates are inserted into messages by SMTP clients or servers
(e.g., in trace fields), four-digit years MUST BE used. Two-digit
years are deprecated; three-digit years were never permitted in the
Internet mail system.

(6) Sending versus mailing

In addition to specifying a mechanism for delivering messages to
user's mailboxes, RFC 821 provided additional, optional, commands to
deliver messages directly to the user's terminal screen. These
commands (SEND, SAML, SOML) were rarely implemented, and changes in
workstation technology and the introduction of other protocols may
have rendered them obsolete even where they are implemented.

Clients SHOULD NOT provide SEND, SAML, or SOML as services. Servers
MAY implement them. If they are implemented by servers, the
implementation model specified in RFC 821 MUST be used and the
command names MUST be published in the response to the EHLO command.

APPENDIX X: Change summary and Loose ends (temporary)

X.1 Change summary

X.1.1 Substantive changes between draft-ietf-drums-smtpupd-00.txt and
draft-ietf-drums-smtpupd-01.txt

(i) Slightly clarified the discussions of rejection and failure of
VRFY requests and the associated response codes.

(ii) Slightly clarified the discussion of deferred address
validation.

(iii) Removed the IPCE terminology and modified the text in section
##4.1.1.2 to explicitly introduce the "mail gateway" terminology and
to begin to distinguish a mail gateway from a conventional relay.
**Please Review This Text**.

(iv) Explicitly noted that SMTP clients for things like POP and IMAP
may send everything to a single relay for further processing, rather
than resolving final domain names.

(v) Tightened the RSET discussion.

(vi) Deprecation of 251 only for RCPT (still ok for VRFY)

X.1.2. Substantive changes between draft-ietf-drums-smtpupd-01.txt
and draft-ietf-drums-smtpupd-02.txt.

Incorporated additional RFC 1123 material; reorganized several
sections for clarity. Added definitions and other previous "loose
end" material.

X.1.3. Substantive changes between draft-ietf-drums-smtpupd-02.txt
and draft-ietf-drums-smtpupd-03.txt.

(i) Eliminated a number of placeholders and tightened some of the
definitions in section 2. Added a few new placeholders for
consistency checking against other documents.

(ii) Removed the state diagrams, per direction at IETF Montreal.

(iii) Added new section 6.3, an attempt to summarize WG discussions
on the "posting" versus "delivery" versus "relay" functions of SMTP
and on whether "fixups" are appropriate in different cases.

(iv) Inserted section 6.1, a minor rewrite of section 5.3.3 of
RFC1123.

(v) Added new text to 3.5.5 to discuss the spammer - EXPN
relationship.

(vi) The "ASCII requirement" in 4.1.1.4 has been tightened somewhat.

(v) The remaining miscellaneous changes agreed to in Montreal have
been incorporated except as noted below.

X.1.4. Substantive changes between draft-ietf-drums-smtpupd-04.txt draft-ietf-drums-smtpupd-03.txt
and draft-ietf-drums-smtpupd-04.txt.

Many small changes have been made between these two versions; the
list that follows is not exhaustive.

(i) To clarify some of the text, definitions have been introduced to
distinguish among originating, delivery, relay, and gateway SMTP
systems.

(ii) The role of LF-terminated lines has been clarified.

(iii) Several changes have been made to clarify the principle
that, no matter what originating and final delivery systems
might do, relay systems are not permitted to tamper with message
content, even to "fix" headers that are determined to be
invalid. If they deem message content to be seriously
unacceptable, they are encouraged to reject the messages in
preference to trying to fix them up, but, in general, the theme
is "don't look/ don't tell".

(iv) A few more definitions have been added to the terminology
section, and the separate glossary has been eliminated.

(v) I have taken a shot at text to address some of the
controversies that have raged on the WG mailing list (e.g.,
sections 7.4 and 7.5). Since there was no consensus on most of
those topics, I expect that the inserted text will satisfy no
one except, perhaps, for agreement that saying nothing would
have been worse. As a mechanism for moving forward, the text in
these controversial areas that now appears will be considered
"base"; alterations will be made only if clear consensus emerges.

(vi) Per discussion in Los Angeles, source routes have been further
deprecated.

(vii) Some of the VRFY/EXPN materials have been moved to "security
considerations", where they appear to belong, some text has been
added, and the conformance statements adjusted to reflect what I
perceive to be WG consensus.

(viii) New MX resolution material has been added to section 5. While
most of this material is from RFC974, added to section 5. While
most of this material is from RFC974, the rules have been further
tightened to reflect current practice and experience (974 is written
in a somewhat speculative fashion for a standard). In particular,
the behavior of trying the target host's A RR when MXs existed but
all of them were eliminated is now prohibited, which seems necessary
if another of other ideas being recommended or considered are to be
feasible.

X.1.5. Substantive changes between draft-ietf-drums-smtpupd-04.txt
and draft-ietf-drums-smtpupd-05.txt.

(i) All normative references to RFC 1123 have been removed from the
main body of the text (some still appear in the appendices where they
will remain).

(ii) Section 3.5 has been renamed slightly to distinguish between
"debuging of SMTP implementations" and "debugging of addresses".
Better terminology would be welcome.

(iii) Error conditions resulting from the DATA command have been
clarified.

(iv) Section 4.2 (SMTP replies) has been revised and tightened to
reflect reality and recent discussion on the list.

(v) Appendix E has been revised a bit and moved into section 4.2.1.
Given the importance of the "check only first digit" rule, it has to
be there.

(vi) Added new text for "no SMTP service supported" to sections
3.1, 4.2.2, 4.2.3, and 4.3.2. As noted in 3.1, I'd rather add 521
(which would work perfectly with the model) rather than overloading
554.

(vii) The Return-path language in section 4.4 has been cleaned up a
bit.

(viii) Tightened the rules have been further
tightened to reflect current practice and experience (974 is written "postmaster" language in 4.5.1, requiring a somewhat speculative fashion
small change to 4.1.1.3.

(ix) I have unilaterally (with a little help from my friends),
increased some of the size limits. 64 was much too short for a standard). In particular,
domain name, and the behavior DNS limit of trying 255 (?) has now been inserted.
That leaves the target host's A RR when MXs existed return path much too short, but
all I haven't fixed it
(maybe that will cause us to get rid of them were eliminated is now prohibited, them). We still have a 64
character limit on the local-part, which seems necessary
if another of other ideas being recommended is also *much* too short.
Votes for 128 or considered longer limits accepted.

(x) The text on the "recipients buffer" has been rewritten so that (I
hope) it makes sense and gives some explicit guidance for how clients
and servers should proceed if limits are to be
feasible. imposed.

X.2 Loose ends

(i) Material in RFC1123, section 5.2.6, not yet fully incorporated.

(ii) Is 5.3.4 of RFC1123 adequately incorporated?

(iii) What needs to be done about RFC1123 5.3.6 and 5.3.7 and where
should it/they go?

(iv) The 822 BNF -> ABNF transition is not yet complete, and most of
what has been done needs checking.

(v)

(ii) Examples not yet revised, overview and grammar still to be
merged.

(vi)

(iii) We have agreed that all of the definition of trace ("Received:")
fields should be moved to this document from the message format one.
That work is not yet complete.

(vii)

(iv) The material in the "Minor special issues" section of RFC 974
really imposes limits on the DNS Appendices have not yet been numbered consecutively. Note
that Appendix X is temporary and is not on the mail system. Should expected to appear in any of that material
final publication.

(v) See X.1.5(ix), above.

(vi) Should the code for "too many recipients" be incorporated into this document? 552 (as in previous
drafts) or 452 (which might make sense, especially given the "first
digit" rule).

(vii) Should we promote the requirement to support timeouts for all
commands to a "MUST"?

See also Chris Newman's "Drums open issues" list.

----