rfc5321
Network Working Group J. Klensin
Request for Comments: 5321 October 2008
Obsoletes: 2821
Updates: 1123
Category: Standards Track
Simple Mail Transfer Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document is a specification of the basic protocol for Internet
electronic mail transport. It consolidates, updates, and clarifies
several previous documents, making all or parts of most of them
obsolete. It covers the SMTP extension mechanisms and best practices
for the contemporary Internet, but does not provide details about
particular extensions. 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 submission"
protocol for "split-UA" (User Agent) mail reading systems and mobile
environments.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Transport of Electronic Mail . . . . . . . . . . . . . . . 5
1.2. History and Context for This Document . . . . . . . . . . 5
1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6
2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 7
2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 9
2.2.1. Background . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2. Definition and Registration of Extensions . . . . . . 10
2.2.3. Special Issues with Extensions . . . . . . . . . . . . 11
2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . . 11
2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . . 11
2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 12
2.3.3. Mail Agents and Message Stores . . . . . . . . . . . . 12
2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . . 13
2.3.6. Buffer and State Table . . . . . . . . . . . . . . . . 14
2.3.7. Commands and Replies . . . . . . . . . . . . . . . . . 14
2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.9. Message Content and Mail Data . . . . . . . . . . . . 15
2.3.10. Originator, Delivery, Relay, and Gateway Systems . . . 15
2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 15
2.4. General Syntax Principles and Transaction Model . . . . . 16
3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . . 17
3.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 18
3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 18
3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 19
3.4. Forwarding for Address Correction or Updating . . . . . . 21
3.5. Commands for Debugging Addresses . . . . . . . . . . . . . 22
3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 22
3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . . 24
3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . . 25
3.5.4. Semantics and Applications of EXPN . . . . . . . . . . 26
3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 26
3.6.1. Source Routes and Relaying . . . . . . . . . . . . . . 26
3.6.2. Mail eXchange Records and Relaying . . . . . . . . . . 26
3.6.3. Message Submission Servers as Relays . . . . . . . . . 27
3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 28
3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 28
3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . . 29
3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 29
3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 29
3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 30
3.8. Terminating Sessions and Connections . . . . . . . . . . . 30
3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 31
3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 31
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3.9.2. List . . . . . . . . . . . . . . . . . . . . . . . . . 31
4. The SMTP Specifications . . . . . . . . . . . . . . . . . . . 32
4.1. SMTP Commands . . . . . . . . . . . . . . . . . . . . . . 32
4.1.1. Command Semantics and Syntax . . . . . . . . . . . . . 32
4.1.2. Command Argument Syntax . . . . . . . . . . . . . . . 41
4.1.3. Address Literals . . . . . . . . . . . . . . . . . . . 43
4.1.4. Order of Commands . . . . . . . . . . . . . . . . . . 44
4.1.5. Private-Use Commands . . . . . . . . . . . . . . . . . 46
4.2. SMTP Replies . . . . . . . . . . . . . . . . . . . . . . . 46
4.2.1. Reply Code Severities and Theory . . . . . . . . . . . 48
4.2.2. Reply Codes by Function Groups . . . . . . . . . . . . 50
4.2.3. Reply Codes in Numeric Order . . . . . . . . . . . . . 52
4.2.4. Reply Code 502 . . . . . . . . . . . . . . . . . . . . 53
4.2.5. Reply Codes after DATA and the Subsequent
<CRLF>.<CRLF> . . . . . . . . . . . . . . . . . . . . 53
4.3. Sequencing of Commands and Replies . . . . . . . . . . . . 54
4.3.1. Sequencing Overview . . . . . . . . . . . . . . . . . 54
4.3.2. Command-Reply Sequences . . . . . . . . . . . . . . . 55
4.4. Trace Information . . . . . . . . . . . . . . . . . . . . 57
4.5. Additional Implementation Issues . . . . . . . . . . . . . 61
4.5.1. Minimum Implementation . . . . . . . . . . . . . . . . 61
4.5.2. Transparency . . . . . . . . . . . . . . . . . . . . . 62
4.5.3. Sizes and Timeouts . . . . . . . . . . . . . . . . . . 62
4.5.3.1. Size Limits and Minimums . . . . . . . . . . . . . 62
4.5.3.1.1. Local-part . . . . . . . . . . . . . . . . . . 63
4.5.3.1.2. Domain . . . . . . . . . . . . . . . . . . . . 63
4.5.3.1.3. Path . . . . . . . . . . . . . . . . . . . . . 63
4.5.3.1.4. Command Line . . . . . . . . . . . . . . . . . 63
4.5.3.1.5. Reply Line . . . . . . . . . . . . . . . . . . 63
4.5.3.1.6. Text Line . . . . . . . . . . . . . . . . . . 63
4.5.3.1.7. Message Content . . . . . . . . . . . . . . . 63
4.5.3.1.8. Recipients Buffer . . . . . . . . . . . . . . 64
4.5.3.1.9. Treatment When Limits Exceeded . . . . . . . . 64
4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . . 64
4.5.3.2. Timeouts . . . . . . . . . . . . . . . . . . . . . 65
4.5.3.2.1. Initial 220 Message: 5 Minutes . . . . . . . . 65
4.5.3.2.2. MAIL Command: 5 Minutes . . . . . . . . . . . 65
4.5.3.2.3. RCPT Command: 5 Minutes . . . . . . . . . . . 65
4.5.3.2.4. DATA Initiation: 2 Minutes . . . . . . . . . . 66
4.5.3.2.5. Data Block: 3 Minutes . . . . . . . . . . . . 66
4.5.3.2.6. DATA Termination: 10 Minutes. . . . . . . . . 66
4.5.3.2.7. Server Timeout: 5 Minutes. . . . . . . . . . . 66
4.5.4. Retry Strategies . . . . . . . . . . . . . . . . . . . 66
4.5.5. Messages with a Null Reverse-Path . . . . . . . . . . 68
5. Address Resolution and Mail Handling . . . . . . . . . . . . . 69
5.1. Locating the Target Host . . . . . . . . . . . . . . . . . 69
5.2. IPv6 and MX Records . . . . . . . . . . . . . . . . . . . 71
6. Problem Detection and Handling . . . . . . . . . . . . . . . . 71
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6.1. Reliable Delivery and Replies by Email . . . . . . . . . . 71
6.2. Unwanted, Unsolicited, and "Attack" Messages . . . . . . . 72
6.3. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 73
6.4. Compensating for Irregularities . . . . . . . . . . . . . 73
7. Security Considerations . . . . . . . . . . . . . . . . . . . 75
7.1. Mail Security and Spoofing . . . . . . . . . . . . . . . . 75
7.2. "Blind" Copies . . . . . . . . . . . . . . . . . . . . . . 76
7.3. VRFY, EXPN, and Security . . . . . . . . . . . . . . . . . 76
7.4. Mail Rerouting Based on the 251 and 551 Response Codes . . 77
7.5. Information Disclosure in Announcements . . . . . . . . . 77
7.6. Information Disclosure in Trace Fields . . . . . . . . . . 78
7.7. Information Disclosure in Message Forwarding . . . . . . . 78
7.8. Resistance to Attacks . . . . . . . . . . . . . . . . . . 78
7.9. Scope of Operation of SMTP Servers . . . . . . . . . . . . 78
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 79
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 80
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 81
10.1. Normative References . . . . . . . . . . . . . . . . . . . 81
10.2. Informative References . . . . . . . . . . . . . . . . . . 82
Appendix A. TCP Transport Service . . . . . . . . . . . . . . . . 85
Appendix B. Generating SMTP Commands from RFC 822 Header
Fields . . . . . . . . . . . . . . . . . . . . . . . 85
Appendix C. Source Routes . . . . . . . . . . . . . . . . . . . . 86
Appendix D. Scenarios . . . . . . . . . . . . . . . . . . . . . . 87
D.1. A Typical SMTP Transaction Scenario . . . . . . . . . . . 88
D.2. Aborted SMTP Transaction Scenario . . . . . . . . . . . . 89
D.3. Relayed Mail Scenario . . . . . . . . . . . . . . . . . . 90
D.4. Verifying and Sending Scenario . . . . . . . . . . . . . . 92
Appendix E. Other Gateway Issues . . . . . . . . . . . . . . . . 92
Appendix F. Deprecated Features of RFC 821 . . . . . . . . . . . 93
F.1. TURN . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
F.2. Source Routing . . . . . . . . . . . . . . . . . . . . . . 93
F.3. HELO . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
F.4. #-literals . . . . . . . . . . . . . . . . . . . . . . . . 94
F.5. Dates and Years . . . . . . . . . . . . . . . . . . . . . 94
F.6. Sending versus Mailing . . . . . . . . . . . . . . . . . . 94
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1. Introduction
1.1. Transport of Electronic Mail
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 [1] describe some of them.
An important feature of SMTP is its capability to transport mail
across multiple networks, usually referred to as "SMTP mail relaying"
(see Section 3.6). A network consists of the mutually-TCP-accessible
hosts on the public Internet, the mutually-TCP-accessible hosts on a
firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
environment utilizing a non-TCP transport-level protocol. Using
SMTP, a process can transfer mail to another process on the same
network or to some other network via a relay or gateway process
accessible to both networks.
In this way, a mail message may pass through a number of intermediate
relay or gateway hosts on its path from sender to ultimate recipient.
The Mail eXchanger mechanisms of the domain name system (RFC 1035
[2], RFC 974 [12], and Section 5 of this document) are used to
identify the appropriate next-hop destination for a message being
transported.
1.2. History and Context for This Document
This document is a specification of the basic protocol for the
Internet electronic mail transport. It consolidates, updates and
clarifies, but does not add new or change existing functionality of
the following:
o the original SMTP (Simple Mail Transfer Protocol) specification of
RFC 821 [1],
o domain name system requirements and implications for mail
transport from RFC 1035 [2] and RFC 974 [12],
o the clarifications and applicability statements in RFC 1123 [3],
and
o material drawn from the SMTP Extension mechanisms in RFC 1869
[13].
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o Editorial and clarification changes to RFC 2821 [14] to bring that
specification to Draft Standard.
It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC
1123 (replacing the mail transport materials of RFC 1123). However,
RFC 821 specifies some features that were not in significant use in
the Internet by the mid-1990s and (in appendices) some additional
transport models. Those sections are omitted here 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 required
amplification. This material has been identified in multiple ways,
mostly by tracking flaming on various lists and newsgroups and
problems of unusual readings or interpretations that have appeared as
the SMTP extensions have been deployed. 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 submission" protocol, as recommended for Post Office
Protocol (POP) (RFC 937 [15], RFC 1939 [16]) and IMAP (RFC 3501
[17]). In general, the separate mail submission protocol specified
in RFC 4409 [18] is now preferred to direct use of SMTP; more
discussion of that subject appears in that document.
Section 2.3 provides definitions of terms specific to this document.
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, RFC 5322 [4], discusses message header sections
and bodies and specifies formats and structures for them.
1.3. Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5]. As each
of these terms was intentionally and carefully chosen to improve the
interoperability of email, each use of these terms is to be treated
as a conformance requirement.
Because this document has a long history and to avoid the risk of
various errors and of confusing readers and documents that point to
this one, most examples and the domain names they contain are
preserved from RFC 2821. Readers are cautioned that these are
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illustrative examples that should not actually be used in either code
or configuration files.
2. The SMTP Model
2.1. Basic Structure
The SMTP design can be pictured as:
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Client- |Commands/Replies| Server- |
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
SMTP client SMTP server
When an SMTP client has a message to transmit, it establishes a two-
way transmission channel to an SMTP server. The responsibility of an
SMTP client is to transfer mail messages to one or more SMTP servers,
or report its failure to do so.
The means by which a mail message is presented to an SMTP client, and
how that client determines the identifier(s) ("names") of the
domain(s) to which mail messages are to be transferred, is a local
matter, and is not addressed by this document. In some cases, the
designated domain(s), or those determined by an SMTP client, will
identify the final destination(s) of the mail message. In other
cases, common with SMTP clients associated with implementations of
the POP (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17])
protocols, or when the SMTP client is inside an isolated transport
service environment, the domain determined will identify an
intermediate destination through which all mail messages are to be
relayed. SMTP clients that transfer all traffic regardless of the
target domains associated with the individual messages, or that do
not maintain queues for retrying message transmissions that initially
cannot be completed, may otherwise conform to this specification but
are not considered fully-capable. Fully-capable SMTP
implementations, including the relays used by these less capable
ones, and their destinations, are expected to support all of the
queuing, retrying, and alternate address functions discussed in this
specification. In many situations and configurations, the less-
capable clients discussed above SHOULD be using the message
submission protocol (RFC 4409 [18]) rather than SMTP.
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The means by which an SMTP client, once it has determined a target
domain, determines the identity of an SMTP server to which a copy of
a message is to be transferred, and then performs that transfer, is
covered by this document. To effect a mail transfer to an SMTP
server, an SMTP client establishes a two-way transmission channel to
that SMTP server. An SMTP client determines the address of an
appropriate host running an SMTP server by resolving a destination
domain name to either an intermediate Mail eXchanger host or a final
target host.
An SMTP server may be either the ultimate destination or an
intermediate "relay" (that is, it may assume the role of an SMTP
client after receiving the message) or "gateway" (that is, it may
transport the message further using some protocol other than SMTP).
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.
In other words, message transfer can occur in a single connection
between the original SMTP-sender and the final SMTP-recipient, or can
occur in a series of hops through intermediary systems. In either
case, once the server has issued a success response at the end of the
mail data, a formal handoff of responsibility for the message occurs:
the protocol requires that a server MUST accept responsibility for
either delivering the message or properly reporting the failure to do
so (see Sections 6.1, 6.2, and 7.8, below).
Once the transmission channel is established and initial handshaking
is 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 lines in the header section 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 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 specifying 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 upon extension
requests such as command pipelining (RFC 2920 [19]).
Once a given mail message has been transmitted, the client may either
request that the connection be shut down or may initiate other mail
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transactions. In addition, an SMTP client may use a connection to an
SMTP server for ancillary services such as verification of email
addresses or retrieval of mailing list subscriber addresses.
As suggested above, this protocol provides mechanisms for the
transmission of mail. Historically, this transmission normally
occurred 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. A very
common case in the Internet today involves submission of the original
message to an intermediate, "message submission" server, which is
similar to a relay but has some additional properties; such servers
are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
An intermediate host that acts as either an SMTP relay or as a
gateway into some other transmission environment is usually selected
through the use of the domain name service (DNS) Mail eXchanger
mechanism.
Usually, intermediate hosts are determined via the DNS MX record, not
by explicit "source" routing (see Section 5 and Appendix C and
Appendix F.2).
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 beyond the original SMTP requirements.
The SMTP extension mechanism defines a means whereby an extended SMTP
client and server may recognize each other, and the server can inform
the client as to the service extensions that it supports.
Contemporary SMTP implementations MUST support the basic extension
mechanisms. For instance, servers MUST support the EHLO command even
if they do not implement any specific extensions and clients SHOULD
preferentially utilize EHLO rather than HELO. (However, for
compatibility with older conforming implementations, SMTP clients and
servers MUST support the original HELO mechanisms as a fallback.)
Unless the different characteristics of HELO must be identified for
interoperability purposes, this document discusses only EHLO.
SMTP is widely deployed and high-quality implementations have proven
to be very robust. However, the Internet community now considers
some services to be important that were not anticipated when the
protocol was first designed. If support for those services is to be
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added, it must be done in a way that permits older implementations to
continue working acceptably. The extension framework consists of:
o The SMTP command EHLO, superseding the earlier HELO,
o a registry of SMTP service extensions,
o additional parameters to the SMTP MAIL and RCPT commands, and
o optional replacements for commands defined in this protocol, such
as for DATA in non-ASCII transmissions (RFC 3030 [20]).
SMTP's strength comes primarily from its simplicity. Experience with
many protocols has shown that protocols with few options tend towards
ubiquity, whereas protocols with many options tend towards obscurity.
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.
2.2.2. Definition and Registration of Extensions
The IANA maintains a registry of SMTP service extensions. A
corresponding EHLO keyword value is associated with each extension.
Each service extension registered with the IANA must be defined in a
formal Standards-Track or IESG-approved Experimental protocol
document. The definition must include:
o the textual name of the SMTP service extension;
o the EHLO keyword value associated with the extension;
o the syntax and possible values of parameters associated with the
EHLO keyword value;
o 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);
o any new parameters the extension associates with the MAIL or RCPT
verbs;
o a description of how support for the extension affects the
behavior of a server and client SMTP; and
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o the increment by which the extension is increasing the maximum
length of the commands MAIL and/or RCPT, over that specified in
this Standard.
In addition, any EHLO keyword value starting with an upper or lower
case "X" refers to a local SMTP service extension used exclusively
through bilateral agreement. Keywords beginning with "X" MUST NOT be
used in a registered service extension. 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 beginning with "X" are local
extensions that may not be registered or standardized. Conversely,
verbs not beginning with "X" must always be registered.
2.2.3. Special Issues with Extensions
Extensions that change fairly basic properties of SMTP operation are
permitted. The text in other sections of this document must be
understood in that context. In particular, extensions can change the
minimum limits specified in Section 4.5.3, can change the ASCII
character set requirement as mentioned above, or can introduce some
optional modes of message handling.
In particular, if an extension implies that the delivery path
normally supports special features of that extension, and an
intermediate SMTP system finds a next hop that does not support the
required extension, it MAY choose, based on the specific extension
and circumstances, to requeue the message and try later and/or try an
alternate MX host. If this strategy is employed, the timeout to fall
back to an unextended format (if one is available) SHOULD be less
than the normal timeout for bouncing as undeliverable (e.g., if
normal timeout is three days, the requeue timeout before attempting
to transmit the mail without the extension might be one day).
2.3. SMTP Terminology
2.3.1. Mail Objects
SMTP transports a mail object. A mail object contains an envelope
and content.
The SMTP envelope is sent as a series of SMTP protocol units
(described in Section 3). It consists of an originator address (to
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which error reports should be directed), one or more recipient
addresses, and optional protocol extension material. Historically,
variations on the reverse-path (originator) address specification
command (MAIL) could be used to specify alternate delivery modes,
such as immediate display; those variations have now been deprecated
(see Appendix F and Appendix F.6).
The SMTP content is sent in the SMTP DATA protocol unit and has two
parts: the header section and the body. If the content conforms to
other contemporary standards, the header section consists of a
collection of header fields, each consisting of a header name, a
colon, and data, structured as in the message format specification
(RFC 5322 [4]); the body, if structured, is defined according to MIME
(RFC 2045 [21]). The content is textual in nature, expressed using
the US-ASCII repertoire [6]. Although SMTP extensions (such as
"8BITMIME", RFC 1652 [22]) may relax this restriction for the content
body, the content header fields are always encoded using the US-ASCII
repertoire. Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
define an algorithm for representing header values outside the US-
ASCII repertoire, while still encoding them using the US-ASCII
repertoire.
2.3.2. Senders 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 and Message Stores
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, e.g., by
depositing the message in a "message store"). 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
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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 the next section); they SHOULD NOT be identified by numerical
addresses, i.e., by address literals as described in Section 4.1.2.
2.3.5. Domain Names
A domain name (or often just a "domain") consists of one or more
components, separated by dots if more than one appears. In the case
of a top-level domain used by itself in an email address, a single
string is used without any dots. This makes the requirement,
described in more detail below, that only fully-qualified domain
names appear in SMTP transactions on the public Internet,
particularly important where top-level domains are involved. These
components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
for SMTP purposes to consist of a sequence of letters, digits, and
hyphens drawn from the ASCII character set [6]. Domain names are
used as names of hosts and of other entities in the domain name
hierarchy. For example, a domain may refer to an alias (label of a
CNAME RR) or the label of Mail eXchanger records to be used to
deliver mail instead of representing a host name. See RFC 1035 [2]
and Section 5 of this specification.
The domain name, as described in this document and in RFC 1035 [2],
is the entire, fully-qualified name (often referred to as an "FQDN").
A domain name that is not in FQDN form is no more than a local alias.
Local aliases MUST NOT appear in any SMTP transaction.
Only resolvable, fully-qualified domain names (FQDNs) are permitted
when domain names are used in SMTP. In other words, names that can
be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
in Section 5) are permitted, as are CNAME RRs whose targets can be
resolved, in turn, to MX or address RRs. Local nicknames or
unqualified names MUST NOT be used. There are two exceptions to the
rule requiring FQDNs:
o The domain name given in the EHLO command MUST be either a primary
host name (a domain name that resolves to an address RR) or, if
the host has no name, an address literal, as described in
Section 4.1.3 and discussed further in the EHLO discussion of
Section 4.1.4.
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o The reserved mailbox name "postmaster" may be used in a RCPT
command without domain qualification (see Section 4.1.1.3) and
MUST be accepted if so used.
2.3.6. Buffer and State Table
SMTP sessions are stateful, with both parties carefully maintaining a
common view of the current state. In this document, we model this
state by a virtual "buffer" and a "state table" on the server that
may be used by the client to, for example, "clear the buffer" or
"reset the state table", causing the information in the buffer to be
discarded and the state to be returned to some previous state.
2.3.7. Commands and Replies
SMTP commands and, unless altered by a service extension, message
data, are transmitted from the sender to the receiver via the
transmission channel in "lines".
An SMTP reply is an acknowledgment (positive or negative) sent in
"lines" from receiver to sender via the transmission channel in
response to a command. The general form of a reply is a numeric
completion code (indicating failure or success) usually followed by a
text string. The codes are for use by programs and the text is
usually intended for human users. RFC 3463 [25], specifies further
structuring of the reply strings, including the use of supplemental
and more specific completion codes (see also RFC 5248 [26]).
2.3.8. Lines
Lines consist of zero or more data characters terminated by the
sequence ASCII character "CR" (hex value 0D) followed immediately by
ASCII character "LF" (hex value 0A). This termination sequence is
denoted as <CRLF> in this document. Conforming implementations MUST
NOT recognize or generate any other character or character sequence
as a line terminator. Limits MAY be imposed on line lengths by
servers (see Section 4).
In addition, the appearance of "bare" "CR" or "LF" characters in text
(i.e., either without the other) has a long history of causing
problems in mail implementations and applications that use the mail
system as a tool. SMTP client implementations MUST NOT transmit
these characters except when they are intended as line terminators
and then MUST, as indicated above, transmit them only as a <CRLF>
sequence.
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2.3.9. Message Content and Mail Data
The terms "message content" and "mail data" are used interchangeably
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 the message header section and
the possibly structured message body. The MIME specification (RFC
2045 [21]) provides the standard mechanisms for structured message
bodies.
2.3.10. Originator, Delivery, Relay, and Gateway Systems
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) introduces mail into the Internet or, more generally,
into a transport service environment. A "delivery" SMTP system is
one that receives mail from a transport service environment and
passes it to a mail user agent or deposits it in a message store that
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.
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. For the purposes of this specification,
firewalls that rewrite addresses should be considered as gateways,
even if SMTP is used on both sides of them (see RFC 2979 [27]).
2.3.11. Mailbox and Address
As used in this specification, an "address" is a character string
that identifies a user to whom mail will be sent or a location into
which mail will be deposited. The term "mailbox" refers to that
depository. The two terms are typically used interchangeably 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". Consequently, and due to a
long history of problems when intermediate hosts have attempted to
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optimize transport by modifying them, the local-part MUST be
interpreted and assigned semantics only by the host specified in the
domain part of the address.
2.4. General Syntax Principles and Transaction Model
SMTP commands and replies have a rigid syntax. All commands begin
with a command verb. All replies begin with a three digit numeric
code. In some commands and replies, arguments are required following
the verb or reply code. Some commands do not accept arguments (after
the verb), and some reply codes are followed, sometimes optionally,
by free form text. In both cases, where text appears, it is
separated from the verb or reply code by a space character. Complete
definitions of commands and replies appear in Section 4.
Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
and extension name keywords) are not case sensitive, with the sole
exception in this specification of a mailbox local-part (SMTP
Extensions may explicitly specify case-sensitive elements). That is,
a command verb, an argument value other than a mailbox local-part,
and free form text MAY be encoded in upper case, lower case, or any
mixture of upper and lower case with no impact on its meaning. The
local-part of a mailbox MUST BE treated as case sensitive.
Therefore, SMTP implementations MUST take care to preserve the case
of mailbox local-parts. In particular, for some hosts, the user
"smith" is different from the user "Smith". However, exploiting the
case sensitivity of mailbox local-parts impedes interoperability and
is discouraged. Mailbox domains follow normal DNS rules and are
hence not case sensitive.
A few SMTP servers, in violation of this specification (and RFC 821)
require that command verbs be encoded by clients in upper case.
Implementations MAY wish to employ this encoding to accommodate those
servers.
The argument clause consists of a variable-length character string
ending with the end of the line, i.e., with the character sequence
<CRLF>. The receiver will 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.
Commands and replies are composed of characters from the ASCII
character set [6]. 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 7-bit
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transport only. An originating SMTP client that has not successfully
negotiated an appropriate extension with a particular server (see the
next paragraph) 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 mislabeled and
the data path cannot accept the actual content, this may result in
the ultimate delivery of a severely garbled message to the recipient.
Delivery SMTP systems MAY reject such messages, or return them as
undeliverable, rather than deliver them. In the absence of a server-
offered extension explicitly permitting it, a sending SMTP system is
not 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.
8-bit message content transmission MAY be requested of the server by
a client using extended SMTP facilities, notably the "8BITMIME"
extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
servers. However, it MUST NOT be construed as authorization to
transmit unrestricted 8-bit material, nor does 8BITMIME authorize
transmission of any envelope material in other than ASCII. 8BITMIME
MUST NOT be requested by senders for material with the high bit on
that is not in MIME format with an appropriate content-transfer
encoding; servers MAY reject such messages.
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 the ABNF
specification in RFC 5234 [7]. Metalanguage terms used in running
text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
The reader is cautioned that the grammar expressed in the
metalanguage is not comprehensive. There are many instances in which
provisions in the text constrain or otherwise modify the syntax or
semantics implied by the grammar.
3. The SMTP Procedures: An Overview
This section contains descriptions of the procedures used in SMTP:
session initiation, mail transaction, forwarding mail, verifying
mailbox names and expanding mailing lists, and opening and closing
exchanges. Comments on relaying, a note on mail domains, and a
discussion of changing roles are included at the end of this section.
Several complete scenarios are presented in Appendix D.
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3.1. Session Initiation
An SMTP session is initiated when a client opens a connection to a
server and the server responds 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, a practice that permits more efficient isolation
and repair of any problems. Implementations MAY make provision for
SMTP servers to disable the software and version announcement where
it causes security concerns. While some systems also identify their
contact point for mail problems, this is not a substitute for
maintaining the required "postmaster" address (see Section 4).
The SMTP protocol allows a server to formally reject a mail session
while still allowing the initial connection as follows: 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 returning
a 554 response on connection opening SHOULD provide enough
information in the reply text to facilitate debugging of the sending
system.
3.2. Client Initiation
Once the server has sent the greeting (welcoming) message and the
client has received it, the client normally sends the EHLO command to
the server, indicating the client's 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 that are unable to
support service extensions, and contemporary clients that do not
require service extensions in the mail session being initiated, MAY
use HELO instead of EHLO. Servers MUST NOT return the extended EHLO-
style response to a HELO command. For a particular connection
attempt, if the server returns a "command not recognized" response to
EHLO, the client SHOULD be able to fall back and send HELO.
In the EHLO 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").
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3.3. Mail Transactions
There are three steps to SMTP mail transactions. The transaction
starts with a MAIL command that gives the sender identification. (In
general, the MAIL command may be sent only when no mail transaction
is in progress; see Section 4.1.4.) A series of one or more RCPT
commands follows, giving the receiver information. Then, a DATA
command initiates transfer of the mail data and is terminated by the
"end of mail" data indicator, which also confirms the transaction.
The first step in the procedure is the MAIL command.
MAIL 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. The <reverse-path> portion of the first or
only argument contains the source mailbox (between "<" and ">"
brackets), 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 MUST return a reply indicating whether the
failure is permanent (i.e., will occur again if the client tries to
send the same address again) or temporary (i.e., the address might be
accepted if the client tries again later). Despite the apparent
scope of this requirement, there are circumstances in which the
acceptability of the reverse-path may not be determined until one or
more forward-paths (in RCPT commands) can be examined. In those
cases, the server MAY reasonably accept the reverse-path (with a 250
reply) and then report problems after the forward-paths are received
and examined. Normally, failures produce 550 or 553 replies.
Historically, the <reverse-path> was permitted to contain more than
just a mailbox; however, 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. This step of
the procedure can be repeated any number of times.
RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
The first or only argument to this command includes a forward-path
(normally a mailbox and domain, always surrounded by "<" and ">"
brackets) identifying one recipient. If accepted, the SMTP server
returns a "250 OK" reply and stores the forward-path. If the
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recipient is known not to be a deliverable address, the SMTP server
returns a 550 reply, typically with a string such as "no such user -
" and the mailbox name (other circumstances and reply codes are
possible).
The <forward-path> can contain more than just a mailbox.
Historically, the <forward-path> was permitted to contain a source
routing list of hosts and the destination mailbox; however,
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 they 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.
These restrictions make a server useless as a relay for clients that
do not support full SMTP functionality. Consequently, restricted-
capability clients MUST NOT assume that any SMTP server on the
Internet can be used as their mail processing (relaying) site. If a
RCPT command appears without a previous MAIL command, the server MUST
return a 503 "Bad sequence of commands" response. The optional
<rcpt-parameters> are associated with negotiated SMTP service
extensions (see Section 2.2).
Since it has been a common source of errors, it is worth noting that
spaces are not permitted on either side of the colon following FROM
in the MAIL command or TO in the RCPT command. The syntax is exactly
as given above.
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 up to but not including the end of
mail data indicator to be the message text. When the end of text is
successfully received and stored, the SMTP-receiver sends a "250 OK"
reply.
Since the mail data is sent on the transmission channel, the end of
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 a "." (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
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data. If accepted, the SMTP server returns a "250 OK" reply. The
DATA command can fail at only two points in the protocol exchange:
If there was no MAIL, or no RCPT, command, or all such commands were
rejected, the server MAY return a "command out of sequence" (503) or
"no valid recipients" (554) reply in response to the DATA command.
If one of those replies (or any other 5yz 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.
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), if resources were unavailable (including, of
course, the server unexpectedly becoming unavailable), or if the
server determines that the message should be rejected for policy or
other reasons.
However, in practice, some servers 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 and, in
particular, in Section 6.1. 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.
When the RFC 822 format ([28], [4]) is being used, the mail data
include the header fields such as those named Date, Subject, To, Cc,
and From. Server SMTP systems SHOULD NOT reject messages based on
perceived defects in the RFC 822 or MIME (RFC 2045 [21]) message
header section or message body. In particular, they MUST NOT reject
messages in which the numbers of Resent-header fields do not match or
Resent-to appears without Resent-from and/or Resent-date.
Mail transaction commands MUST be used in the order discussed above.
3.4. Forwarding for Address Correction or Updating
Forwarding support is most often required to consolidate and simplify
addresses within, or relative to, some enterprise and less frequently
to establish addresses to link a person's prior address with a
current one. Silent forwarding of messages (without server
notification to the sender), for security or non-disclosure purposes,
is common in the contemporary Internet.
In both the enterprise and the "new address" cases, information
hiding (and sometimes security) considerations argue against exposure
of the "final" address through the SMTP protocol as a side effect of
the forwarding activity. This may be especially important when the
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final address may not even be reachable by the sender. Consequently,
the "forwarding" mechanisms described in Section 3.2 of RFC 821, and
especially the 251 (corrected destination) and 551 reply codes from
RCPT must be evaluated carefully by implementers and, when they are
available, by those configuring systems (see also Section 7.4).
In particular:
o Servers MAY forward messages when they are aware of an address
change. When they do so, they MAY either provide address-updating
information with a 251 code, or may forward "silently" and return
a 250 code. However, if a 251 code is used, they MUST NOT assume
that the client will actually update address information or even
return that information to the user.
Alternately,
o Servers MAY reject messages or return them as non-deliverable when
they cannot be delivered precisely as addressed. When they do so,
they MAY either provide address-updating information with a 551
code, or may reject the message as undeliverable with a 550 code
and no address-specific information. However, if a 551 code is
used, they MUST NOT assume that the client will actually update
address information or even return that information to the user.
SMTP server implementations that support the 251 and/or 551 reply
codes SHOULD provide configuration mechanisms so that sites that
conclude that they would undesirably disclose information can disable
or restrict their use.
3.5. Commands for Debugging Addresses
3.5.1. Overview
SMTP provides commands to verify a user name or obtain the content of
a mailing list. This is done with the VRFY and EXPN commands, which
have character string arguments. Implementations SHOULD support VRFY
and EXPN (however, see Section 3.5.2 and Section 7.3).
For the VRFY command, the string is a user name or a user name and
domain (see below). If a normal (i.e., 250) response is returned,
the response MAY include the full name of the user and MUST include
the mailbox of the user. It MUST be in either of the following
forms:
User Name <local-part@domain>
local-part@domain
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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, any of the following are legitimate
responses to VRFY:
553 User ambiguous
or
553- Ambiguous; Possibilities are
553-Joe Smith <jsmith@foo.com>
553-Harry Smith <hsmith@foo.com>
553 Melvin Smith <dweep@foo.com>
or
553-Ambiguous; Possibilities
553- <jsmith@foo.com>
553- <hsmith@foo.com>
553 <dweep@foo.com>
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, such as those described in RFC
3463 [25], will facilitate automated translation into other languages
as needed. Of course, a client that was highly automated or that was
operating in another language than English might choose to try to
translate the response to return some other indication to the user
than the literal text of the reply, or to take some automated action
such as consulting a directory service for additional information
before reporting to the user.
For the EXPN command, the string identifies a mailing list, and the
successful (i.e., 250) multiline response MAY include the full name
of the users and MUST give the mailboxes on the mailing list.
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 containing only one mailbox. If a request is made to apply
VRFY to a mailing list, a positive response MAY be given if a message
so addressed would be delivered to everyone on the list, otherwise an
error SHOULD be reported (e.g., "550 That is a mailing list, not a
user" or "252 Unable to verify members of mailing list"). If a
request is made to expand a user name, the server MAY return a
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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 successful 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.
"User name" is a fuzzy term and has been used deliberately. An
implementation of the VRFY or EXPN commands MUST include at least
recognition of local mailboxes as "user names". 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 "local-part@domain" form as a "user
name"; hosts MAY also choose to recognize other strings as "user
names".
The case of expanding a mailbox list requires a multiline reply, such
as:
C: EXPN Example-People
S: 250-Jon Postel <Postel@isi.edu>
S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
or
C: EXPN Executive-Washroom-List
S: 550 Access Denied to You.
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 are a variety
of file naming conventions in the Internet. Similarly, historical
variations in what is returned by these commands are such that the
response SHOULD be interpreted very carefully, if at all, and SHOULD
generally only be used for diagnostic purposes.
3.5.2. VRFY Normal Response
When normal (2yz or 551) responses are returned from a VRFY or EXPN
request, the reply MUST include the <Mailbox> name using a
"<local-part@domain>" construction, where "domain" is a fully-
qualified domain name. In circumstances exceptional enough to
justify violating the intent of this specification, free-form text
MAY be returned. In order to facilitate parsing by both computers
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and people, addresses SHOULD appear in pointed brackets. When
addresses, rather than free-form debugging information, are returned,
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. Paths (explicit source routes) MUST NOT be
returned by VRFY or EXPN.
Server implementations SHOULD support both VRFY and 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 (see Section 7.3). 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, but VRFY was
made mandatory in RFC 1123 [3], if EXPN is supported, it MUST be
listed as a service extension in an EHLO response. VRFY MAY be
listed as a convenience but, since support for it is required, SMTP
clients are not required to check for its presence on the extension
list before using it.
3.5.3. Meaning of VRFY or EXPN Success Response
A server MUST NOT return a 250 code in response to a VRFY or EXPN
command unless it has actually verified the address. In particular,
a server MUST NOT return 250 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. As
stated elsewhere, implementation (in the sense of actually validating
addresses and returning information) of VRFY and EXPN are strongly
recommended. Hence, implementations that return 500 or 502 for VRFY
are not in full compliance with this specification.
There may be circumstances where an address appears to be valid but
cannot reasonably be verified in real time, particularly when a
server is acting as a mail exchanger for another server or domain.
"Apparent validity", in this case, would normally involve at least
syntax checking and might involve verification that any domains
specified were ones to 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 in Section 2.1.
Similarly, the discussion in Section 3.4 applies to the use of reply
codes 251 and 551 with VRFY (and EXPN) to indicate addresses that are
recognized but that would be forwarded or rejected were mail received
for them. Implementations generally SHOULD be more aggressive about
address verification in the case of VRFY than in the case of RCPT,
even if it takes a little longer to do so.
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3.5.4. Semantics and Applications of EXPN
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 for hosts (typically with MX and CNAME DNS
records), for mailboxes (various types of local host aliases), and in
various proxying arrangements has made it nearly impossible for these
strategies to work consistently, and mail systems SHOULD NOT attempt
them.
3.6. Relaying and Mail Routing
3.6.1. Source Routes and Relaying
In general, the availability of Mail eXchanger records in the domain
name system (RFC 1035 [2], RFC 974 [12]) makes the use of explicit
source routes in the Internet mail system unnecessary. Many
historical problems with the interpretation of explicit source routes
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. When route information is encountered, SMTP
servers MAY ignore the route information and simply send to the final
destination specified as the last element in the route and SHOULD do
so. There has been an invalid practice 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. This is one of several reasons why SMTP clients MUST NOT
generate invalid source routes or depend on serial resolution of
names.
When 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.
3.6.2. Mail eXchange Records and Relaying
A relay SMTP server is usually the target of a DNS MX record that
designates it, rather than the final delivery system. 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 it declines to
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relay mail to a particular address for policy reasons, a 550 response
SHOULD be returned.
This specification does not deal with the verification of return
paths for use in delivery notifications. Recent work, such as that
on SPF [29] and DKIM [30] [31], has been done to provide ways to
ascertain that an address is valid or belongs to the person who
actually sent the message. A server MAY attempt to verify the return
path before using its address for delivery notifications, but methods
of doing so are not defined here nor is any particular method
recommended at this time.
3.6.3. Message Submission Servers as Relays
Many mail-sending clients exist, especially in conjunction with
facilities that receive mail via POP3 or IMAP, that have limited
capability to support some of the requirements of this specification,
such as the ability to queue messages for subsequent delivery
attempts. For these clients, it is common practice to make private
arrangements to send all messages to a single server for processing
and subsequent distribution. SMTP, as specified here, is not ideally
suited for this role. A standardized mail submission protocol has
been developed that is gradually superseding practices based on SMTP
(see RFC 4409 [18]). In any event, because these arrangements are
private and fall outside the scope of this specification, they are
not described here.
It is important to note that MX records can point to SMTP servers
that act as gateways into other environments, not just SMTP relays
and final delivery systems; see Sections 3.7 and 5.
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
(see, for example, RFC 3461 [32] and RFC 3464 [33]) 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 transporting 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 (see
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Section 4.5.5 for additional discussion). A MAIL command with a null
reverse-path appears as follows:
MAIL FROM:<>
As discussed in Section 6.4, a relay SMTP has no need to inspect or
act upon the header section or body of the message data and MUST NOT
do so except to add its own "Received:" header field (Section 4.4)
and, optionally, to attempt to detect looping in the mail system (see
Section 6.3). Of course, this prohibition also applies to any
modifications of these header fields or text (see also Section 7.9).
3.7. Mail Gatewaying
While the relay function discussed above operates within the Internet
SMTP transport service environment, MX records or various forms of
explicit routing may require that an intermediate SMTP server perform
a translation function between one transport service and another. As
discussed in Section 2.3.10, when such a system is at the boundary
between two transport service environments, we refer to it as a
"gateway" or "gateway SMTP".
Gatewaying mail between different mail environments, such as
different mail formats and protocols, is complex and does not easily
yield to standardization. However, some general requirements may be
given for a gateway between the Internet and another mail
environment.
3.7.1. Header Fields in Gatewaying
Header fields MAY be rewritten when necessary as messages are
gatewayed across mail environment boundaries. This may involve
inspecting the message body or interpreting the local-part of the
destination address in spite of the prohibitions in Section 6.4.
Other mail systems gatewayed to the Internet often use a subset of
the RFC 822 header section or provide similar functionality with a
different syntax, but some of these mail systems do not have an
equivalent to the SMTP envelope. Therefore, when a message leaves
the Internet environment, it may be necessary to fold the SMTP
envelope information into the message header section. 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 programs in foreign environments and
might risk disclosure of private information (see Section 7.2).
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3.7.2. Received Lines in Gatewaying
When forwarding a message into or out of the 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 header section.
"Received:" header fields of messages originating from other
environments may not conform exactly to this specification. However,
the most important use of Received: lines is for debugging mail
faults, and this debugging can be severely hampered by well-meaning
gateways that try to "fix" a Received: line. As another consequence
of trace header fields arising in non-SMTP environments, receiving
systems MUST NOT reject mail based on the format of a trace header
field and SHOULD be extremely robust in the light of unexpected
information or formats in those header fields.
The gateway SHOULD indicate the environment and protocol in the "via"
clauses of Received header field(s) that it supplies.
3.7.3. Addresses in Gatewaying
From the Internet side, the gateway SHOULD accept all valid address
formats in SMTP commands and in the RFC 822 header section, and all
valid RFC 822 messages. Addresses and header fields generated by
gateways MUST conform to applicable standards (including this one and
RFC 5322 [4]). Gateways are, of course, subject to the same rules
for handling source routes as those described for other SMTP systems
in Section 3.3.
3.7.4. Other Header Fields in Gatewaying
The gateway MUST ensure that all header fields of a message that it
forwards into the Internet mail environment meet the requirements for
Internet mail. In particular, all addresses in "From:", "To:",
"Cc:", etc., header fields MUST be transformed (if necessary) to
satisfy the standard header syntax of RFC 5322 [4], MUST reference
only fully-qualified domain names, and MUST be effective and useful
for sending replies. The translation algorithm used to convert mail
from the Internet protocols to another environment's protocol SHOULD
ensure that error messages from the foreign mail environment are
delivered to the reverse-path from the SMTP envelope, not to an
address in the "From:", "Sender:", or similar header fields of the
message.
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3.7.5. Envelopes in Gatewaying
Similarly, when forwarding a message from another environment into
the Internet, the gateway SHOULD set the envelope return path in
accordance with an error message return address, if supplied by the
foreign environment. If the foreign environment has no equivalent
concept, the gateway must select and use a best approximation, with
the message originator's address as the default of last resort.
3.8. Terminating Sessions and Connections
An SMTP connection is terminated when the client sends a QUIT
command. The server responds with a positive reply code, after which
it closes the connection.
An SMTP server MUST NOT intentionally close the connection under
normal operational circumstances (see Section 7.8) except:
o After receiving a QUIT command and responding with a 221 reply.
o After detecting the need to shut down the SMTP service and
returning a 421 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 the
client will receive it after the next command is issued).
o After a timeout, as specified in Section 4.5.3.2, occurs waiting
for the client to send a command or data.
In particular, a server that closes connections in response to
commands that are not understood is in violation of this
specification. Servers are expected to be tolerant of unknown
commands, issuing a 500 reply and awaiting further instructions from
the client.
An SMTP server that is forcibly shut down via external means SHOULD
attempt to send a line containing a 421 response code to the SMTP
client before exiting. The SMTP client will normally read the 421
response code after sending its next command.
SMTP clients that experience a connection close, reset, or 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
act accordingly.
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3.9. Mailing Lists and Aliases
An SMTP-capable host SHOULD support both the alias and the list
models of address expansion for multiple delivery. When a message is
delivered or forwarded to each address of an expanded list form, the
return address in the envelope ("MAIL FROM:") MUST be changed to be
the address of a person or other entity who administers the list.
However, in this case, the message header section (RFC 5322 [4]) MUST
be left unchanged; in particular, the "From" field of the header
section is unaffected.
An important mail facility is a mechanism for multi-destination
delivery of a single message, by transforming (or "expanding" or
"exploding") a pseudo-mailbox address into a list of destination
mailbox addresses. When a message is sent to such a pseudo-mailbox
(sometimes called an "exploder"), copies are forwarded or
redistributed to each mailbox in the expanded list. Servers SHOULD
simply utilize the addresses on the list; application of heuristics
or other matching rules to eliminate some addresses, such as that of
the originator, is strongly discouraged. We classify such a pseudo-
mailbox as an "alias" or a "list", depending upon the expansion
rules.
3.9.1. Alias
To expand an alias, the recipient mailer simply replaces the pseudo-
mailbox address in the envelope with each of the expanded addresses
in turn; the rest of the envelope and the message body are left
unchanged. The message is then delivered or forwarded to each
expanded address.
3.9.2. List
A mailing list may be said to operate by "redistribution" rather than
by "forwarding". To expand a list, the recipient mailer replaces the
pseudo-mailbox address in the envelope with each of the expanded
addresses in turn. The return (backward-pointing) address in the
envelope is changed so that all error messages generated by the final
deliveries will be returned to a list administrator, not to the
message originator, who generally has no control over the contents of
the list and will typically find error messages annoying. Note that
the key difference between handling aliases (Section 3.9.1) and
forwarding (this subsection) is the change to the backward-pointing
address in this case. When a list constrains its processing to the
very limited set of modifications and actions described here, it is
attempting to emulate an MTA; such lists can be treated as a
continuation in email transit.
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There exist mailing lists that perform additional, sometimes
extensive, modifications to a message and its envelope. Such mailing
lists need to be viewed as full MUAs, which accept a delivery and
post a new message.
4. The SMTP Specifications
4.1. SMTP Commands
4.1.1. Command Semantics and Syntax
The SMTP commands define the mail transfer or the mail system
function requested by the user. SMTP commands are character strings
terminated by <CRLF>. The commands themselves are alphabetic
characters terminated by <SP> if parameters follow and <CRLF>
otherwise. (In the interest of improved interoperability, SMTP
receivers SHOULD tolerate trailing white space before the terminating
<CRLF>.) The syntax of the local part of a mailbox MUST conform to
receiver site conventions and the syntax 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 that are
communicated as arguments to different commands. The reverse-path is
the argument of the MAIL command, the forward-path is the argument of
the RCPT command, and the mail data is the argument of the DATA
command. These arguments or data objects must be transmitted and
held, pending the confirmation communicated by the end of mail data
indication that finalizes the 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 buffer. Specific commands cause information to be appended
to a specific buffer, or cause one or more buffers to be cleared.
Several commands (RSET, DATA, QUIT) are specified as not permitting
parameters. In the absence of specific extensions offered by the
server and accepted by the client, clients MUST NOT send such
parameters and servers SHOULD reject commands containing them as
having invalid syntax.
4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO)
These commands are used to identify the SMTP client to the SMTP
server. The argument clause contains the fully-qualified domain name
of the SMTP client, if one is available. In situations 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
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available), the client SHOULD send an address literal (see
Section 4.1.3).
RFC 2821, and some earlier informal practices, encouraged following
the literal by information that would help to identify the client
system. That convention was not widely supported, and many SMTP
servers considered it an error. In the interest of interoperability,
it is probably wise for servers to be prepared for this string to
occur, but SMTP clients SHOULD NOT send it.
The SMTP server identifies itself to the SMTP client in the
connection greeting reply and in the response to this command.
A client SMTP SHOULD start an SMTP session by issuing the EHLO
command. If the SMTP server supports the SMTP service extensions, it
will give a successful response, a failure response, or an error
response. If 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, and servers MUST
support the HELO command and reply properly to it. In any event, a
client MUST issue HELO or EHLO before starting a mail transaction.
These commands, and a "250 OK" reply to one of them, confirm that
both the SMTP client and the SMTP server are in the initial state,
that is, there is no transaction in progress and all state tables and
buffers are cleared.
Syntax:
ehlo = "EHLO" SP ( Domain / address-literal ) CRLF
helo = "HELO" SP Domain CRLF
Normally, the response to EHLO will be a multiline reply. Each line
of the response contains a keyword and, optionally, one or more
parameters. Following the normal syntax for multiline replies, these
keywords follow the code (250) and a hyphen for all but the last
line, and the code and a space for the last line. The syntax for a
positive response, using the ABNF notation and terminal symbols of
RFC 5234 [7], is:
ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
/ ( "250-" Domain [ SP ehlo-greet ] CRLF
*( "250-" ehlo-line CRLF )
"250" SP ehlo-line CRLF )
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ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127)
; string of any characters other than CR or LF
ehlo-line = ehlo-keyword *( SP ehlo-param )
ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; additional syntax of ehlo-params depends on
; ehlo-keyword
ehlo-param = 1*(%d33-126)
; any CHAR excluding <SP> and all
; control characters (US-ASCII 0-31 and 127
; inclusive)
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.
The EHLO response MUST contain keywords (and associated parameters if
required) for all commands not listed as "required" in Section 4.5.1
excepting only private-use commands as described in Section 4.1.5.
Private-use commands MAY be listed.
4.1.1.2. MAIL (MAIL)
This command is used to initiate a mail transaction in which the mail
data is delivered to an SMTP server that may, in turn, deliver it to
one or more mailboxes or pass it on to another system (possibly using
SMTP). The argument clause contains a reverse-path and may contain
optional parameters. In general, the MAIL command may be sent only
when no mail transaction is in progress, see Section 4.1.4.
The reverse-path consists of the sender mailbox. Historically, that
mailbox might optionally have been preceded by a list of hosts, but
that behavior is now deprecated (see Appendix C). In some types of
reporting messages for which a reply is likely to cause a mail loop
(for example, mail delivery and non-delivery notifications), the
reverse-path may be null (see Section 3.6).
This command clears the reverse-path buffer, the forward-path buffer,
and the mail data buffer, and it inserts the reverse-path information
from its argument clause into the reverse-path buffer.
If service extensions were negotiated, the MAIL command may also
carry parameters associated with a particular service extension.
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Syntax:
mail = "MAIL FROM:" Reverse-path
[SP Mail-parameters] CRLF
4.1.1.3. RECIPIENT (RCPT)
This command is used to identify an individual recipient of the mail
data; multiple recipients are specified by multiple uses of this
command. The argument clause contains a forward-path and may contain
optional parameters.
The forward-path normally consists of the required destination
mailbox. Sending systems SHOULD NOT generate the optional list of
hosts known as a source route. Receiving systems MUST recognize
source route syntax but SHOULD strip off the source route
specification and utilize the domain name associated with the mailbox
as if the source route had not been provided.
Similarly, relay hosts SHOULD strip or ignore source routes, and
names MUST NOT be copied into the reverse-path. When mail reaches
its ultimate destination (the forward-path contains only a
destination mailbox), the SMTP server inserts it into the destination
mailbox in accordance with its host mail conventions.
This command appends its forward-path argument to the forward-path
buffer; it does not change the reverse-path buffer nor the mail data
buffer.
For example, mail received at relay host xyz.com with envelope
commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
will normally be sent directly on to host d.bar.org with envelope
commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<userc@d.bar.org>
As provided in Appendix C, xyz.com MAY also choose to relay the
message to hosta.int, using the envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
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or to jkl.org, using the envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@jkl.org:userc@d.bar.org>
Attempting to use relaying this way is now strongly discouraged.
Since hosts are not required to relay mail at all, xyz.com MAY also
reject the message entirely when the RCPT command is received, using
a 550 code (since this is a "policy reason").
If service extensions were negotiated, the RCPT command may also
carry parameters associated with a particular service extension
offered by the server. The client MUST NOT transmit parameters other
than those associated with a service extension offered by the server
in its EHLO response.
Syntax:
rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>" /
Forward-path ) [SP Rcpt-parameters] CRLF
Note that, in a departure from the usual rules for
local-parts, the "Postmaster" string shown above is
treated as case-insensitive.
4.1.1.4. DATA (DATA)
The receiver normally sends a 354 response to DATA, and then treats
the lines (strings ending in <CRLF> sequences, as described in
Section 2.3.7) following the command as mail data from the sender.
This command causes the mail data to be appended to the mail data
buffer. The mail data may contain any of the 128 ASCII character
codes, although experience has indicated that use of control
characters other than SP, HT, CR, and LF may cause problems and
SHOULD be avoided when possible.
The mail data are terminated by a line containing only a period, that
is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is
actually the terminator of the previous line (see Section 4.5.2).
This is the end of mail data indication. The first <CRLF> of this
terminating sequence is also the <CRLF> that ends the final line of
the data (message text) or, if there was no mail data, ends the DATA
command itself (the "no mail data" case does not conform to this
specification since it would require that neither the trace header
fields required by this specification nor the message header section
required by RFC 5322 [4] be transmitted). An extra <CRLF> MUST NOT
be added, as that would cause an empty line to be added to the
message. The only exception to this rule would arise if the message
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body were passed to the originating SMTP-sender with a final "line"
that did not end in <CRLF>; in that case, the originating SMTP system
MUST either reject the message as invalid or add <CRLF> in order to
have the receiving SMTP server recognize the "end of data" condition.
The custom of accepting lines ending only in <LF>, as a concession 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 in the name of improved
robustness. In particular, the sequence "<LF>.<LF>" (bare line
feeds, without carriage returns) MUST NOT be treated as equivalent to
<CRLF>.<CRLF> as the end of mail data indication.
Receipt of the end of mail data indication requires the server to
process the stored mail transaction information. This processing
consumes the information in the reverse-path buffer, the forward-path
buffer, and the mail data buffer, and on the completion of this
command these buffers are cleared. If the processing is successful,
the receiver MUST send an OK reply. If the processing fails, the
receiver MUST send a failure reply. The SMTP model does not allow
for partial failures at this point: either the message is accepted by
the server for delivery and a positive response is returned or it is
not accepted and a failure reply is returned. In sending a positive
"250 OK" completion reply to the end of data indication, the receiver
takes full responsibility for the message (see Section 6.1). Errors
that are diagnosed subsequently MUST be reported in a mail message,
as discussed in Section 4.4.
When the SMTP server accepts a message either for relaying or for
final delivery, it inserts a trace record (also referred to
interchangeably as a "time stamp line" or "Received" line) at the top
of the mail data. This trace record indicates the identity of the
host that sent the message, the identity of the host that received
the message (and is inserting this time stamp), and the date 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 in Section 4.4.
Additional discussion about the operation of the DATA command appears
in Section 3.3.
Syntax:
data = "DATA" CRLF
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4.1.1.5. RESET (RSET)
This command specifies that the current mail transaction will be
aborted. Any stored sender, recipients, and mail data MUST be
discarded, and all buffers and state tables cleared. The receiver
MUST send a "250 OK" reply to a RSET command with no arguments. A
reset command may be issued by the client at any time. It is
effectively equivalent to a NOOP (i.e., it has no effect) if issued
immediately after EHLO, before EHLO is issued in the session, after
an end of data indicator has been sent and acknowledged, or
immediately before a QUIT. 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).
Since EHLO implies some additional processing and response by the
server, RSET will normally be more efficient than reissuing that
command, even though the formal semantics are the same.
There are circumstances, contrary to the intent of this
specification, in which an SMTP server may receive an indication that
the underlying TCP connection has been closed or reset. To preserve
the robustness of the mail system, SMTP servers SHOULD be prepared
for this condition and SHOULD treat it as if a QUIT had been received
before the connection disappeared.
Syntax:
rset = "RSET" CRLF
4.1.1.6. VERIFY (VRFY)
This command asks the receiver to confirm that the argument
identifies a user or mailbox. If it is a user name, information is
returned as specified in Section 3.5.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer.
Syntax:
vrfy = "VRFY" SP String CRLF
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4.1.1.7. EXPAND (EXPN)
This command asks the receiver to confirm that the argument
identifies a mailing list, and if so, to return the membership of
that list. If the command is successful, a reply is returned
containing information as described in Section 3.5. This reply will
have multiple lines except in the trivial case of a one-member list.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer, and it may be issued at any
time.
Syntax:
expn = "EXPN" SP String CRLF
4.1.1.8. HELP (HELP)
This command causes the server to send helpful information to the
client. 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 the reverse-path buffer, the forward-
path buffer, or the mail data buffer, and it may be issued at any
time.
SMTP servers SHOULD support HELP without arguments and MAY support it
with arguments.
Syntax:
help = "HELP" [ SP String ] CRLF
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4.1.1.9. NOOP (NOOP)
This command does not affect any parameters or previously entered
commands. It specifies no action other than that the receiver send a
"250 OK" reply.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer, and it may be issued at any
time. If a parameter string is specified, servers SHOULD ignore it.
Syntax:
noop = "NOOP" [ SP String ] CRLF
4.1.1.10. QUIT (QUIT)
This command specifies that the receiver MUST send a "221 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 (even if there was an
error). The sender MUST NOT intentionally close the transmission
channel until it sends a QUIT command, and it SHOULD wait until it
receives the reply (even if there was an error response to a previous
command). If the connection is closed prematurely due to violations
of the above or system or network failure, the server MUST cancel any
pending transaction, but not undo any previously completed
transaction, and generally MUST act as if the command or transaction
in progress had received a temporary error (i.e., a 4yz response).
The QUIT command may be issued at any time. Any current uncompleted
mail transaction will be aborted.
Syntax:
quit = "QUIT" CRLF
4.1.1.11. Mail-Parameter and Rcpt-Parameter Error Responses
If the server SMTP does not recognize or cannot implement one or more
of the parameters associated with a particular MAIL FROM or RCPT TO
command, it will return code 555.
If, for some reason, the server is temporarily unable to accommodate
one or more of the parameters associated with a MAIL FROM or RCPT TO
command, and if the definition of the specific parameter does not
mandate the use of another code, it should return code 455.
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Errors specific to particular parameters and their values will be
specified in the parameter's defining RFC.
4.1.2. Command Argument Syntax
The syntax of the argument clauses of the above commands (using the
syntax specified in RFC 5234 [7] where applicable) is given below.
Some of the productions given below are used only in conjunction with
source routes as described in Appendix C. Terminals not defined in
this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined
in the "core" syntax in Section 6 of RFC 5234 [7] or in the message
format syntax in RFC 5322 [4].
Reverse-path = Path / "<>"
Forward-path = Path
Path = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l = At-domain *( "," At-domain )
; Note that this form, the so-called "source
; route", MUST BE accepted, SHOULD NOT be
; generated, and SHOULD be ignored.
At-domain = "@" Domain
Mail-parameters = esmtp-param *(SP esmtp-param)
Rcpt-parameters = esmtp-param *(SP esmtp-param)
esmtp-param = esmtp-keyword ["=" esmtp-value]
esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
esmtp-value = 1*(%d33-60 / %d62-126)
; any CHAR excluding "=", SP, and control
; characters. If this string is an email address,
; i.e., a Mailbox, then the "xtext" syntax [32]
; SHOULD be used.
Keyword = Ldh-str
Argument = Atom
Domain = sub-domain *("." sub-domain)
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sub-domain = Let-dig [Ldh-str]
Let-dig = ALPHA / DIGIT
Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
address-literal = "[" ( IPv4-address-literal /
IPv6-address-literal /
General-address-literal ) "]"
; See Section 4.1.3
Mailbox = Local-part "@" ( Domain / address-literal )
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*atext
Quoted-string = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
; i.e., backslash followed by any ASCII
; graphic (including itself) or SPace
qtextSMTP = %d32-33 / %d35-91 / %d93-126
; i.e., within a quoted string, any
; ASCII graphic or space is permitted
; without blackslash-quoting except
; double-quote and the backslash itself.
String = Atom / Quoted-string
While the above definition for Local-part is relatively permissive,
for maximum interoperability, a host that expects to receive mail
SHOULD avoid defining mailboxes where the Local-part requires (or
uses) the Quoted-string form or where 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 MUST
be treated as equivalent, and the sending system SHOULD transmit the
form that uses the minimum quoting possible.
Systems MUST NOT define mailboxes in such a way as to require the use
in SMTP of non-ASCII characters (octets with the high order bit set
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to one) or ASCII "control characters" (decimal value 0-31 and 127).
These characters MUST NOT be used in MAIL or RCPT commands or other
commands that require mailbox names.
Note that the backslash, "\", is a quote character, which is used to
indicate that the next character is to be used literally (instead of
its normal interpretation). For example, "Joe\,Smith" indicates a
single nine-character user name string with the comma being the
fourth character of that string.
To promote interoperability and consistent with long-standing
guidance about conservative use of the DNS in naming and applications
(e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [2]),
characters outside the set of alphabetic characters, digits, and
hyphen MUST NOT appear in domain name labels for SMTP clients or
servers. In particular, the underscore character is not permitted.
SMTP servers that receive a command in which invalid character codes
have been employed, and for which there are no other reasons for
rejection, MUST reject that command with a 501 response (this rule,
like others, could be overridden by appropriate SMTP extensions).
4.1.3. Address Literals
Sometimes a host is not known to the domain name system and
communication (and, in particular, communication to report and repair
the error) is blocked. To bypass this barrier, a special literal
form of the address is allowed as an alternative to a domain name.
For IPv4 addresses, this form uses four small decimal integers
separated by dots and enclosed by brackets such as [123.255.37.2],
which indicates an (IPv4) Internet Address in sequence-of-octets
form. For IPv6 and other forms of addressing that might eventually
be standardized, the form consists of a standardized "tag" that
identifies the address syntax, a colon, and the address itself, in a
format specified as part of the relevant standards (i.e., RFC 4291
[8] for IPv6).
Specifically:
IPv4-address-literal = Snum 3("." Snum)
IPv6-address-literal = "IPv6:" IPv6-addr
General-address-literal = Standardized-tag ":" 1*dcontent
Standardized-tag = Ldh-str
; Standardized-tag MUST be specified in a
; Standards-Track RFC and registered with IANA
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dcontent = %d33-90 / ; Printable US-ASCII
%d94-126 ; excl. "[", "\", "]"
Snum = 1*3DIGIT
; representing a decimal integer
; value in the range 0 through 255
IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
IPv6-hex = 1*4HEXDIG
IPv6-full = IPv6-hex 7(":" IPv6-hex)
IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::"
[IPv6-hex *5(":" IPv6-hex)]
; The "::" represents at least 2 16-bit groups of
; zeros. No more than 6 groups in addition to the
; "::" may be present.
IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-liter
