Network Working Group J. Klensin, Editor
Request for Comments: 2821 AT&T Laboratories
Obsoletes: 821, 974, 1869 April 2001
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.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document is a self-contained specification of the basic protocol
for the Internet electronic mail transport. It consolidates, updates
and clarifies, but doesn't add new or change existing functionality
of the following:
- the original SMTP (Simple Mail Transfer Protocol) specification of
RFC 821 [30],
- domain name system requirements and implications for mail
transport from RFC 1035 [22] and RFC 974 [27],
- the clarifications and applicability statements in RFC 1123 [2],
and
- material drawn from the SMTP Extension mechanisms [19].
It obsoletes RFC 821, RFC 974, and updates RFC 1123 (replaces 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.
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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 POP [3, 26]
and IMAP 
. Additional submission issues are discussed in RFC 2476
[15].
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 [32] discusses message headers, message bodies
and formats and structures for them, and their relationship.
Table of Contents
1. Introduction .................................................. 4
2. The SMTP Model ................................................ 5
2.1 Basic Structure .............................................. 5
2.2 The Extension Model .......................................... 7
2.2.1 Background ................................................. 7
2.2.2 Definition and Registration of Extensions .................. 8
2.3 Terminology .................................................. 9
2.3.1 Mail Objects ............................................... 10
2.3.2 Senders and Receivers ...................................... 10
2.3.3 Mail Agents and Message Stores ............................. 10
2.3.4 Host ....................................................... 11
2.3.5 Domain ..................................................... 11
2.3.6 Buffer and State Table ..................................... 11
2.3.7 Lines ...................................................... 12
2.3.8 Originator, Delivery, Relay, and Gateway Systems ........... 12
2.3.9 Message Content and Mail Data .............................. 13
2.3.10 Mailbox and Address ....................................... 13
2.3.11 Reply ..................................................... 13
2.4 General Syntax Principles and Transaction Model .............. 13
3. The SMTP Procedures: An Overview .............................. 15
3.1 Session Initiation ........................................... 15
3.2 Client Initiation ............................................ 16
3.3 Mail Transactions ............................................ 16
3.4 Forwarding for Address Correction or Updating ................ 19
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3.5 Commands for Debugging Addresses ............................. 20
3.5.1 Overview ................................................... 20
3.5.2 VRFY Normal Response ....................................... 22
3.5.3 Meaning of VRFY or EXPN Success Response ................... 22
3.5.4 Semantics and Applications of EXPN ......................... 23
3.6 Domains ...................................................... 23
3.7 Relaying ..................................................... 24
3.8 Mail Gatewaying .............................................. 25
3.8.1 Header Fields in Gatewaying ................................ 26
3.8.2 Received Lines in Gatewaying ............................... 26
3.8.3 Addresses in Gatewaying .................................... 26
3.8.4 Other Header Fields in Gatewaying .......................... 27
3.8.5 Envelopes in Gatewaying .................................... 27
3.9 Terminating Sessions and Connections ......................... 27
3.10 Mailing Lists and Aliases ................................... 28
3.10.1 Alias ..................................................... 28
3.10.2 List ...................................................... 28
4. The SMTP Specifications ....................................... 29
4.1 SMTP Commands ................................................ 29
4.1.1 Command Semantics and Syntax ............................... 29
4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO) ................... 29
4.1.1.2 MAIL (MAIL) .............................................. 31
4.1.1.3 RECIPIENT (RCPT) ......................................... 31
4.1.1.4 DATA (DATA) .............................................. 33
4.1.1.5 RESET (RSET) ............................................. 34
4.1.1.6 VERIFY (VRFY) ............................................ 35
4.1.1.7 EXPAND (EXPN) ............................................ 35
4.1.1.8 HELP (HELP) .............................................. 35
4.1.1.9 NOOP (NOOP) .............................................. 35
4.1.1.10 QUIT (QUIT) ............................................. 36
4.1.2 Command Argument Syntax .................................... 36
4.1.3 Address Literals ........................................... 38
4.1.4 Order of Commands .......................................... 39
4.1.5 Private-use Commands ....................................... 40
4.2 SMTP Replies ................................................ 40
4.2.1 Reply Code Severities and Theory ........................... 42
4.2.2 Reply Codes by Function Groups ............................. 44
4.2.3 Reply Codes in Numeric Order .............................. 45
4.2.4 Reply Code 502 ............................................. 46
4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF> .... 46
4.3 Sequencing of Commands and Replies ........................... 47
4.3.1 Sequencing Overview ........................................ 47
4.3.2 Command-Reply Sequences .................................... 48
4.4 Trace Information ............................................ 49
4.5 Additional Implementation Issues ............................. 53
4.5.1 Minimum Implementation ..................................... 53
4.5.2 Transparency ............................................... 53
4.5.3 Sizes and Timeouts ......................................... 54
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4.5.3.1 Size limits and minimums ................................. 54
4.5.3.2 Timeouts ................................................. 56
4.5.4 Retry Strategies ........................................... 57
4.5.4.1 Sending Strategy ......................................... 58
4.5.4.2 Receiving Strategy ....................................... 59
4.5.5 Messages with a null reverse-path .......................... 59
5. Address Resolution and Mail Handling .......................... 60
6. Problem Detection and Handling ................................ 62
6.1 Reliable Delivery and Replies by Email ....................... 62
6.2 Loop Detection ............................................... 63
6.3 Compensating for Irregularities .............................. 63
7. Security Considerations ....................................... 64
7.1 Mail Security and Spoofing ................................... 64
7.2 "Blind" Copies ............................................... 65
7.3 VRFY, EXPN, and Security ..................................... 65
7.4 Information Disclosure in Announcements ...................... 66
7.5 Information Disclosure in Trace Fields ....................... 66
7.6 Information Disclosure in Message Forwarding ................. 67
7.7 Scope of Operation of SMTP Servers ........................... 67
8. IANA Considerations ........................................... 67
9. References .................................................... 68
10. Editor's Address ............................................. 70
11. Acknowledgments .............................................. 70
Appendices ....................................................... 71
A. TCP Transport Service ......................................... 71
B. Generating SMTP Commands from RFC 822 Headers ................. 71
C. Source Routes ................................................. 72
D. Scenarios ..................................................... 73
E. Other Gateway Issues .......................................... 76
F. Deprecated Features of RFC 821 ................................ 76
Full Copyright Statement ......................................... 79
1. Introduction
The objective of the Simple Mail Transfer Protocol (SMTP) is to
transfer mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and
requires only a reliable ordered data stream channel. While this
document specifically discusses transport over TCP, other transports
are possible. Appendices to RFC 821 describe some of them.
An important feature of SMTP is its capability to transport mail
across networks, usually referred to as "SMTP mail relaying" (see
section 3.8). 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
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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 [22, 27] (and
section 5 of this document) are used to identify the appropriate
next-hop destination for a message being transported.
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 domain name(s) to which mail messages
are to be transferred is a local matter, and is not addressed by this
document. In some cases, the domain name(s) transferred to, or
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 [3, 26] or IMAP
protocols, or when the SMTP client is inside an isolated transport
service environment, the domain name 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 domain names 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
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ones, and their destinations, are expected to support all of the
queuing, retrying, and alternate address functions discussed in this
specification.
The means by which an SMTP client, once it has determined a target
domain name, 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, a formal handoff of responsibility for the message occurs: the
protocol requires that a server accept responsibility for either
delivering a message or properly reporting the failure to do so.
Once the transmission channel is established and initial handshaking
completed, the SMTP client normally initiates a mail transaction.
Such a transaction consists of a series of commands to specify the
originator and destination of the mail and transmission of the
message content (including any headers or other structure) itself.
When the same message is sent to multiple recipients, this protocol
encourages the transmission of only one copy of the data for all
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 extension requests such as command
pipelining [13].
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Once a given mail message has been transmitted, the client may either
request that the connection be shut down or may initiate other mail
transactions. 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. This transmission normally occurs directly
from the sending user's host to the receiving user's host when the
two hosts are connected to the same transport service. When they are
not connected to the same transport service, transmission occurs via
one or more relay SMTP servers. An intermediate host that 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 appendices C and
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
added, it must be done in a way that permits older implementations to
continue working acceptably. The extension framework consists of:
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- The SMTP command EHLO, superseding the earlier HELO,
- a registry of SMTP service extensions,
- additional parameters to the SMTP MAIL and RCPT commands, and
- optional replacements for commands defined in this protocol, such
as for DATA in non-ASCII transmissions [33].
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:
- the textual name of the SMTP service extension;
- the EHLO keyword value associated with the extension;
- the syntax and possible values of parameters associated with the
EHLO keyword value;
- 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);
- any new parameters the extension associates with the MAIL or RCPT
verbs;
- a description of how support for the extension affects the
behavior of a server and client SMTP; and,
- the increment by which the extension is increasing the maximum
length of the commands MAIL and/or RCPT, over that specified in
this standard.
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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.
2.3 Terminology
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 below.
1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that
the definition is an absolute requirement of the specification.
2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the
definition is an absolute prohibition of the specification.
3. SHOULD This word, or the adjective "RECOMMENDED", mean that
there may exist valid reasons in particular circumstances to
ignore a particular item, but the full implications must be
understood and carefully weighed before choosing a different
course.
4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean
that there may exist valid reasons in particular circumstances
when the particular behavior is acceptable or even useful, but the
full implications should be understood and the case carefully
weighed before implementing any behavior described with this
label.
5. MAY This word, or the adjective "OPTIONAL", mean that an item is
truly optional. One vendor may choose to include the item because
a particular marketplace requires it or because the vendor feels
that it enhances the product while another vendor may omit the
same item. An implementation which does not include a particular
option MUST be prepared to interoperate with another
implementation which does include the option, though perhaps with
reduced functionality. In the same vein an implementation which
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does include a particular option MUST be prepared to interoperate
with another implementation which does not include the option
(except, of course, for the feature the option provides.)
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
which error reports should be directed); one or more recipient
addresses; and optional protocol extension material. Historically,
variations on the recipient address specification command (RCPT TO)
could be used to specify alternate delivery modes, such as immediate
display; those variations have now been deprecated (see appendix F,
section F.6).
The SMTP content is sent in the SMTP DATA protocol unit and has two
parts: the headers and the body. If the content conforms to other
contemporary standards, the headers form a collection of field/value
pairs structured as in the message format specification [32]; the
body, if structured, is defined according to MIME [12]. The content
is textual in nature, expressed using the US-ASCII repertoire [1].
Although SMTP extensions (such as "8BITMIME" [20]) may relax this
restriction for the content body, the content headers are always
encoded using the US-ASCII repertoire. A MIME extension [23] defines
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
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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
Internet mail. Hence, the reader should be cautious about inferring
the strong relationships and responsibilities that might be implied
if these terms were used elsewhere.
2.3.4 Host
For the purposes of this specification, a host is a computer system
attached to the Internet (or, in some cases, to a private TCP/IP
network) and supporting the SMTP protocol. Hosts are known by names
(see "domain"); identifying them by numerical address is discouraged.
2.3.5 Domain
A domain (or domain name) consists of one or more dot-separated
components. These components ("labels" in DNS terminology [22]) are
restricted for SMTP purposes to consist of a sequence of letters,
digits, and hyphens drawn from the ASCII character set [1]. 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 [22] and
section 5 of this specification.
The domain name, as described in this document and in [22], 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.
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 which
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.
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2.3.7 Lines
SMTP commands and, unless altered by a service extension, message
data, are transmitted in "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 4.5.3).
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.
2.3.8 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
which a mail user agent is expected to subsequently access. A
"relay" SMTP system (usually referred to just as a "relay") receives
mail from an SMTP client and transmits it, without modification to
the message data other than adding trace information, to another SMTP
server for further relaying or for delivery.
A "gateway" SMTP system (usually referred to just as a "gateway")
receives mail from a client system in one transport environment and
transmits it to a server system in another transport environment.
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 [11]).
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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 message headers and the
possibly-structured message body. The MIME specification [12]
provides the standard mechanisms for structured message bodies.
2.3.10 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
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.
2.3.11 Reply
An SMTP reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a
command. The general form of a reply is a 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. Recent work [34] has specified further structuring
of the reply strings, including the use of supplemental and more
specific completion codes.
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 MUST follow 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.
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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. This
is NOT true of a mailbox local-part. 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. Mailbox
domains are not case sensitive. 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.
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 field 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 [1]. When the transport service provides an 8-bit byte
(octet) transmission channel, each 7-bit character is transmitted
right justified in an octet with the high order bit cleared to zero.
More specifically, the unextended SMTP service provides seven bit
transport only. An originating SMTP client which has not
successfully negotiated an appropriate extension with a particular
server 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 ultimate delivery of a
severely garbled message to the recipient. Delivery SMTP systems MAY
reject ("bounce") such messages rather than deliver them. No sending
SMTP system is permitted to send envelope commands in any character
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set other than US-ASCII; receiving systems SHOULD reject such
commands, normally using "500 syntax error - invalid character"
replies.
Eight-bit message content transmission MAY be requested of the server
by a client using extended SMTP facilities, notably the "8BITMIME"
extension [20]. 8BITMIME SHOULD be supported by SMTP servers.
However, it MUST not be construed as authorization to transmit
unrestricted eight bit material. 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 
. Metalanguage terms used in running text are
surrounded by pointed brackets (e.g., <CRLF>) for clarity.
3. The SMTP Procedures: An Overview
This section contains descriptions of the procedures used in SMTP:
session initiation, the mail transaction, forwarding mail, verifying
mailbox names and expanding mailing lists, and the 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.
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 4.5.1).
The SMTP protocol allows a server to formally reject a transaction
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
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"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 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 which are unable to
support service extensions and contemporary clients which 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").
3.3 Mail Transactions
There are three steps to SMTP mail transactions. The transaction
starts with a MAIL command which 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
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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> can 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.
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 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).
This step of the procedure can be repeated any number of times.
The <forward-path> can contain more than just a mailbox.
Historically, the <forward-path> can be 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 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).
The third step in the procedure is the DATA command (or some
alternative specified in a service extension).
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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
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), or 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. 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 RFC 822 format [7, 32] is being used, the mail data include the
memo header items such as Date, Subject, To, Cc, From. Server SMTP
systems SHOULD NOT reject messages based on perceived defects in the
RFC 822 or MIME [12] message header or message body. In particular,
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they MUST NOT reject messages in which the numbers of Resent-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 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
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.
In particular:
* 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. But, 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,
* Servers MAY reject or bounce messages when they are not
deliverable when 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. But, 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 are strongly encouraged to provide configuration mechanisms so
that sites which conclude that they would undesirably disclose
information can disable or restrict their use.
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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 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
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
response 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 [34],
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
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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
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.
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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 normally includes the mailbox name, i.e.,
"<local-part@domain>", where "domain" is a fully qualified domain
name, MUST appear in the syntax. 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
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. When these commands are supported, they
are not required to work across relays when relaying is supported.
Since they were both optional in RFC 821, they MUST be listed as
service extensions in an EHLO response, if they are supported.
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.
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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 discussed 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 bounced
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.
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 Domains
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 A RRs (as discussed in section 5) are
permitted, as are CNAME RRs whose targets can be resolved, in turn,
to MX or A RRs. Local nicknames or unqualified names MUST NOT be
used. There are two exceptions to the rule requiring FQDNs:
- The domain name given in the EHLO command MUST BE either a primary
host name (a domain name that resolves to an A RR) or, if the host
has no name, an address literal as described in section 4.1.1.1.
- 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.
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3.7 Relaying
In general, the availability of Mail eXchanger records in the domain
name system [22, 27] makes the use of explicit source routes in the
Internet mail system unnecessary. Many historical problems with
their interpretation have made their use undesirable. SMTP clients
SHOULD NOT generate explicit source routes except under unusual
circumstances. SMTP servers MAY decline to act as mail relays or to
accept addresses that specify source routes. When route information
is encountered, SMTP servers are also permitted to 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.
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
relay mail to a particular address for policy reasons, a 550 response
SHOULD be returned.
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, and work is underway on standardized mail
submission protocols that might eventually supercede the current
practices. In any event, because these arrangements are private and
fall outside the scope of this specification, they are not described
here.
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It is important to note that MX records can point to SMTP servers
which act as gateways into other environments, not just SMTP relays
and final delivery systems; see sections 3.8 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, [24, 25]) 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 section
4.5.5 for additional discussion). A MAIL command with a null
reverse-path appears as follows:
MAIL FROM:<>
As discussed in section 2.4.1, a relay SMTP has no need to inspect or
act upon the headers or body of the message data and MUST NOT do so
except to add its own "Received:" header (section 4.4) and,
optionally, to attempt to detect looping in the mail system (see
section 6.2).
3.8 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 2.3.8, 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.
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3.8.1 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 2.4.1.
Other mail systems gatewayed to the Internet often use a subset of
RFC 822 headers 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. 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).
3.8.2 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.
"Received:" 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
fields arising in non-SMTP environments, receiving systems MUST NOT
reject mail based on the format of a trace field and SHOULD be
extremely robust in the light of unexpected information or formats in
those fields.
The gateway SHOULD indicate the environment and protocol in the "via"
clauses of Received field(s) that it supplies.
3.8.3 Addresses in Gatewaying
From the Internet side, the gateway SHOULD accept all valid address
formats in SMTP commands and in RFC 822 headers, and all valid RFC
822 messages. Addresses and headers generated by gateways MUST
conform to applicable Internet standards (including this one and RFC
822). Gateways are, of course, subject to the same rules for
handling source routes as those described for other SMTP systems in
section 3.3.
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3.8.4 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., fields MUST be transformed (if necessary) to satisfy RFC
822 syntax, 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 return path from the
SMTP envelope, not to the sender listed in the "From:" field (or
other fields) of the RFC 822 message.
3.8.5 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.
3.9 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 except:
- After receiving a QUIT command and responding with a 221 reply.
- 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).
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.
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An SMTP server which 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 ro