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 22.214.171.124) 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
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
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> 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).
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
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
- 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  message header or message body. In particular,
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-
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.
* 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.
* 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.
3.5 Commands for Debugging Addresses
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
User Name <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
553- Ambiguous; Possibilities are
553-Joe Smith <email@example.com>
553-Harry Smith <firstname.lastname@example.org>
553 Melvin Smith <email@example.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 ,
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
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
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
The case of expanding a mailbox list requires a multiline reply, such
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>
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 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.
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
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 126.96.36.199.
- The reserved mailbox name "postmaster" may be used in a RCPT
command without domain qualification (see section 188.8.131.52) and
MUST be accepted if so used.
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
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:
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
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
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
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
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
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 robustness of the mail system,
treat the mail transaction as if a 451 response had been received and
3.10 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  MUST be left
unchanged; in particular, the "From" field of the message header is
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
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
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 all of the expanded
addresses. The return 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.
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 are encouraged to 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 which 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 which 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.
184.108.40.206 Extended HELLO (EHLO) or HELLO (HELO)
These commands are used to identify the SMTP client to the SMTP
server. The argument field 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
available), the client SHOULD send an address literal (see section
4.1.3), optionally followed by information that will help to identify
the client system. y The SMTP server identifies itself to the SMTP
client in the connection greeting reply and in the response to this
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.
ehlo = "EHLO" SP Domain 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
keyworks 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
ehlo-ok-rsp = ( "250" domain [ SP ehlo-greet ] CRLF )
/ ( "250-" domain [ SP ehlo-greet ] CRLF
*( "250-" ehlo-line CRLF )
"250" SP ehlo-line CRLF )
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-param = 1*(%d33-127)
; any CHAR excluding <SP> and all
; control characters (US-ASCII 0-31 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 2.4.1.
220.127.116.11 MAIL (MAIL)
This command is used to initiate a mail transaction in which the mail
data is delivered to an SMTP server which may, in turn, deliver it to
one or more mailboxes or pass it on to another system (possibly using
SMTP). The argument field 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 nondelivery notifications), the
reverse-path may be null (see section 3.7).
This command clears the reverse-path buffer, the forward-path buffer,
and the mail data buffer; and inserts the reverse-path information
from this command into the reverse-path buffer.
If service extensions were negotiated, the MAIL command may also
carry parameters associated with a particular service extension.
"MAIL FROM:" ("<>" / Reverse-Path)
[SP Mail-parameters] CRLF
18.104.22.168 RECIPIENT (RCPT)
This command is used to identify an individual recipient of the mail
data; multiple recipients are specified by multiple use of this
command. The argument field contains a forward-path and may contain
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.
For example, mail received at relay host xyz.com with envelope
will normally be sent directly on to host d.bar.org with envelope
As provided in appendix C, xyz.com MAY also choose to relay the
message to hosta.int, using the envelope commands
or to jkl.org, using the envelope commands
Of course, 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.
"RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>" / Forward-Path)
[SP Rcpt-parameters] CRLF
22.214.171.124 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 is terminated by a line containing only a period, that
is, the character sequence "<CRLF>.<CRLF>" (see section 4.5.2). This
is the end of mail data indication. Note that 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 data, ends the DATA
command itself. 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 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
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.
126.96.36.199 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., if 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 188.8.131.52).
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.
184.108.40.206 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.
"VRFY" SP String CRLF
220.127.116.11 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 may be issued at any time.
"EXPN" SP String CRLF
18.104.22.168 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 may be issued at any time.
SMTP servers SHOULD support HELP without arguments and MAY support it
"HELP" [ SP String ] CRLF
22.214.171.124 NOOP (NOOP)
This command does not affect any parameters or previously entered
commands. It specifies no action other than that the receiver send
an OK reply.
This command has no effect on the reverse-path buffer, the forward-
path buffer, or the mail data buffer and may be issued at any time.
If a parameter string is specified, servers SHOULD ignore it.
"NOOP" [ SP String ] CRLF
126.96.36.199 QUIT (QUIT)
This command specifies that the receiver MUST send an OK reply, and
then close the transmission channel.
The receiver MUST NOT intentionally close the transmission channel
until it receives and replies to a QUIT command (even if there was an
error). The sender MUST NOT intentionally close the transmission
channel until it sends a QUIT command and 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.
4.1.2 Command Argument Syntax
The syntax of the argument fields of the above commands (using the
syntax specified in  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 [8 (section 6)] or in the message format syntax
Reverse-path = Path
Forward-path = Path
Path = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l = At-domain *( "," A-d-l )
; Note that this form, the so-called "source route",
; MUST BE accepted, SHOULD NOT be generated, and SHOULD be
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 / %d62-127)
; any CHAR excluding "=", SP, and control characters
Keyword = Ldh-str
Argument = Atom
Domain = (sub-domain 1*("." sub-domain)) / address-literal
sub-domain = Let-dig [Ldh-str]
address-literal = "[" IPv4-address-literal /
; See section 4.1.3
Mailbox = Local-part "@" Domain
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*atext
Quoted-string = DQUOTE *qcontent DQUOTE
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
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 field with the comma being the fourth
character of the field.
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, RFC1035 ),
characters outside the set of alphas, 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.
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 [188.8.131.52], 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 IPv6 standards .
IPv4-address-literal = Snum 3("." Snum)
IPv6-address-literal = "IPv6:" IPv6-addr
General-address-literal = Standardized-tag ":" 1*dcontent
Standardized-tag = Ldh-str
; MUST be specified in a standards-track RFC
; and registered with IANA
Snum = 1*3DIGIT ; representing a decimal integer
; value in the range 0 through 255
Let-dig = ALPHA / DIGIT
Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
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(":"
; The "::" represents at least 2 16-bit groups of zeros
; No more than 6 groups in addition to the "::" may be
IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::"
[IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal
; The "::" represents at least 2 16-bit groups of zeros
; No more than 4 groups in addition to the "::" and
; IPv4-address-literal may be present
4.1.4 Order of Commands
There are restrictions on the order in which these commands may be
A session that will contain mail transactions MUST first be
initialized by the use of the EHLO command. An SMTP server SHOULD
accept commands for non-mail transactions (e.g., VRFY or EXPN)
without this initialization.
An EHLO command MAY be issued by a client later in the session. If
it is issued after the session begins, the SMTP server MUST clear all
buffers and reset the state exactly as if a RSET command had been
issued. In other words, the sequence of RSET followed immediately by
EHLO is redundant, but not harmful other than in the performance cost
of executing unnecessary commands.
If the EHLO command is not acceptable to the SMTP server, 501, 500,
or 502 failure replies MUST be returned as appropriate. The SMTP
server MUST stay in the same state after transmitting these replies
that it was in before the EHLO was received.
The SMTP client MUST, if possible, ensure that the domain parameter
to the EHLO command is a valid principal host name (not a CNAME or MX
name) for its host. If this is not possible (e.g., when the client's
address is dynamically assigned and the client does not have an
obvious name), an address literal SHOULD be substituted for the
domain name and supplemental information provided that will assist in
identifying the client.
An SMTP server MAY verify that the domain name parameter in the EHLO
command actually corresponds to the IP address of the client.
However, the server MUST NOT refuse to accept a message for this
reason if the verification fails: the information about verification
failure is for logging and tracing only.
The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
during a session, or without previously initializing a session. SMTP
servers SHOULD process these normally (that is, not return a 503
code) even if no EHLO command has yet been received; clients SHOULD
open a session with EHLO before sending these commands.
If these rules are followed, the example in RFC 821 that shows "550
access denied to you" in response to an EXPN command is incorrect
unless an EHLO command precedes the EXPN or the denial of access is
based on the client's IP address or other authentication or
The MAIL command (or the obsolete SEND, SOML, or SAML commands)
begins a mail transaction. Once started, a mail transaction consists
of a transaction beginning command, one or more RCPT commands, and a
DATA command, in that order. A mail transaction may be aborted by
the RSET (or a new EHLO) command. There may be zero or more
transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be
sent if a mail transaction is already open, i.e., it should be sent
only if no mail transaction had been started in the session, or it
the previous one successfully concluded with a successful DATA
command, or if the previous one was aborted with a RSET.
If the transaction beginning command argument is not acceptable, a
501 failure reply MUST be returned and the SMTP server MUST stay in
the same state. If the commands in a transaction are out of order to
the degree that they cannot be processed by the server, a 503 failure
reply MUST be returned and the SMTP server MUST stay in the same
The last command in a session MUST be the QUIT command. The QUIT
command cannot be used at any other time in a session, but SHOULD be
used by the client SMTP to request connection closure, even when no
session opening command was sent and accepted.
4.1.5 Private-use Commands
As specified in section 2.2.2, commands starting in "X" may be used
by bilateral agreement between the client (sending) and server
(receiving) SMTP agents. An SMTP server that does not recognize such
a command is expected to reply with "500 Command not recognized". An
extended SMTP server MAY list the feature names associated with these
private commands in the response to the EHLO command.
Commands sent or accepted by SMTP systems that do not start with "X"
MUST conform to the requirements of section 2.2.2.