Internet Engineering Task Force (IETF) R. Gellens
Request for Comments: 6409 QUALCOMM Incorporated
STD: 72 J. Klensin
Obsoletes: 4409 November 2011
Category: Standards Track
Message Submission for Mail
This memo splits message submission from message relay, allowing each
service to operate according to its own rules (for security, policy,
etc.), and specifies what actions are to be taken by a submission
Message relay is unaffected, and continues to use SMTP over port 25.
When conforming to this document, message submission uses the
protocol specified here, normally over port 587.
This separation of function offers a number of benefits, including
the ability to apply specific security or policy requirements.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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SMTP [SMTP-MTA] was defined as a message *transfer* protocol, that
is, a means to route (if needed) and deliver finished (complete)
Message Transfer Agents (MTAs) are not supposed to alter the message
text, except to add 'Received', 'Return-Path', and other header
fields as required by [SMTP-MTA]. However, SMTP is now also widely
used as a message *submission* protocol, that is, a means for Message
User Agents (MUAs) to introduce new messages into the MTA routing
network. The process that accepts message submissions from MUAs is
termed a "Message Submission Agent" (MSA).
In order to permit unconstrained communications, SMTP is not often
authenticated during message relay.
Authentication and authorization of initial submissions have become
increasingly important, driven by changes in security requirements
and rising expectations that submission servers take responsibility
for the message traffic they originate.
For example, due to the prevalence of machines that have worms,
viruses, or other malicious software that generate large amounts of
spam, many sites now prohibit outbound traffic on the standard SMTP
port (port 25), funneling all mail submissions through submission
In addition to authentication and authorization issues, messages
being submitted are, in some cases, finished (complete) messages and,
in other cases, are unfinished (incomplete) in one or more aspects.
Unfinished messages may need to be completed to ensure they conform
to the Message Format specification [MESSAGE-FORMAT] and related
requirements. For example, the message may lack a proper 'Date'
header field, and domains might not be fully qualified. In some
cases, the MUA may be unable to generate finished messages (e.g., it
might not know its time zone). Even when submitted messages are
complete, local site policy may dictate that the message text be
examined or modified in some way, e.g., to conceal local name or
address spaces. Such completions or modifications have been shown to
cause harm when performed by downstream MTAs -- that is, MTAs after
the first-hop submission MTA -- and are, in general, considered to be
outside the province of standardized MTA functionality.
Separating messages into submissions and transfers allows developers
and network administrators to do the following more easily:
o Implement security policies and guard against unauthorized mail
relaying or injection of unsolicited bulk mail.
o Implement authenticated submission, including off-site submission
by authorized users such as travelers.
o Separate the relevant software code differences, thereby making
each code base more straightforward and allowing for different
programs for relay and submission.
o Detect configuration problems with a site's mail clients.
o Provide a basis for adding enhanced submission services.
This memo describes a low-cost, deterministic means for messages to
be identified as submissions, and it specifies what actions are to be
taken by a submission server.
2. Document Information
2.1. Definitions of Terms Used in This Memo
Many of the concepts and terms used in this document are defined in
[SMTP-MTA]; familiarity with those documents is assumed here.
Containing or consisting of a domain that can be globally resolved
using the Domain Name Service, that is, not a local alias or partial
Message Submission Agent (MSA)
A process that conforms to this specification. An MSA acts as a
submission server to accept messages from MUAs, and it either
delivers them or acts as an SMTP client to relay them to an MTA.
Message Transfer Agent (MTA)
A process that conforms to [SMTP-MTA]. An MTA acts as an SMTP server
to accept messages from an MSA or another MTA, and it either delivers
them or acts as an SMTP client to relay them to another MTA.
Message User Agent (MUA)
A process that acts (often on behalf of a user and with a user
interface) to compose and submit new messages, and to process
For delivered messages, the receiving MUA may obtain and process the
message according to local conventions or, in what is commonly
referred to as a split-MUA model, Post Office Protocol [POP3] or IMAP
[IMAP4] is used to access delivered messages, whereas the protocol
defined here (or SMTP) is used to submit messages.
2.2. Conventions Used in This Document
Examples use the 'example.net' domain.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in [KEYWORDS].
3. Message Submission
3.1. Submission Identification
Port 587 is reserved for email message submission as specified in
this document. Messages received on this port are defined to be
submissions. The protocol used is ESMTP [SMTP-MTA], with additional
restrictions or allowances as specified here.
Although most email clients and servers can be configured to use port
587 instead of 25, there are cases where this is not possible or
convenient. A site MAY choose to use port 25 for message submission
by designating some hosts to be MSAs and others to be MTAs.
3.2. Message Rejection and Bouncing
MTAs and MSAs MAY implement message rejection rules that rely, in
part, on whether the message is a submission or a relay.
For example, some sites might configure their MTAs to reject all RCPT
commands for messages that do not reference local users, and they
might configure their MSA to reject all message submissions that do
not come from authorized users, with authorization based on either
the authenticated identity or the submitting endpoint being within a
protected IP environment.
NOTE: It is better to reject a message than to risk sending one that
is damaged. This is especially true for problems that are
correctable by the MUA, for example, an invalid 'From' field.
If an MSA is not able to determine a return path to the submitting
user, from a valid MAIL FROM, a valid source IP address, or based on
authenticated identity, then the MSA SHOULD immediately reject the
message. A message can be immediately rejected by returning a 550
code to the MAIL command.
Note that a null return path, that is, MAIL FROM:<>, is permitted and
MUST NOT, in itself, be cause for rejecting a message. (MUAs need to
generate null return-path messages for a variety of reasons,
including disposition notifications.)
Except in the case where the MSA is unable to determine a valid
return path for the message being submitted, text in this
specification that instructs an MSA to issue a rejection code MAY be
complied with by accepting the message and subsequently generating a
bounce message. (That is, if the MSA is going to reject a message
for any reason except being unable to determine a return path, it can
optionally do an immediate rejection or accept the message and then
mail a bounce.)
NOTE: In the normal case of message submission, immediately rejecting
the message is preferred, as it gives the user and MUA direct
feedback. To properly handle delayed bounces, the client MUA needs
to maintain a queue of messages it has submitted and match bounces to
them. Note that many contemporary MUAs do not have this capability.
3.3. Authorized Submission
Numerous methods have been used to ensure that only authorized users
are able to submit messages. These methods include authenticated
SMTP, IP address restrictions, secure IP and other tunnels, and prior
Authenticated SMTP [SMTP-AUTH] has seen widespread deployment. It
allows the MSA to determine an authorization identity for the message
submission, one that is not tied to other protocols.
IP address restrictions are very widely implemented, but they do not
allow for travelers and similar situations, and they can be easily
spoofed unless all transport paths between the MUA and MSA are
Secure IP [IPSEC], and other encrypted and authenticated tunneling
techniques, can also be used and provide additional benefits of
protection against eavesdropping and traffic analysis.
Requiring a POP [POP3] authentication (from the same IP address)
within some amount of time (e.g., 20 minutes) prior to the start of a
message submission session has also been used, but this does impose
restrictions on clients as well as servers, which may cause
difficulties. Specifically, the client must do a POP authentication
before an SMTP submission session, and not all clients are capable
and configured for this. Also, the MSA must coordinate with the POP
server, which may be difficult. There is also a window during which
an unauthorized user can submit messages and appear to be a
previously authorized user. Since it is dependent on the MUA's IP
addresses, this technique is substantially as subject to IP address
spoofing as validation based on known IP addresses alone (see above).
4. Mandatory Actions
An MSA MUST do all of the following:
4.1. General Submission Rejection Code
Unless covered by a more precise response code, response code 554 is
to be used to reject a MAIL, RCPT, or DATA command that contains
4.2. Ensure All Domains Are Fully Qualified
The MSA MUST ensure that all domains in the SMTP envelope are fully
If the MSA examines or alters the message text in any way, except to
add trace header fields [SMTP-MTA], it MUST ensure that all domains
in address header fields are fully qualified.
Reply code 554 is to be used to reject a MAIL, RCPT, or DATA command
that contains improper domain references.
A frequent local convention is to accept single-level domains (e.g.,
'sales') and then to expand the reference by adding the remaining
portion of the domain name (e.g., to 'sales.example.net'). Local
conventions that permit single-level domains SHOULD reject, rather
than expand, incomplete multi-level domains (e.g., 'squeaky.sales'),
since such expansion is particularly risky.
4.3. Require Authentication
The MSA MUST, by default, issue an error response to the MAIL command
if the session has not been authenticated using [SMTP-AUTH], unless
it has already independently established authentication or
authorization (such as being within a protected subnetwork).
Section 3.3 discusses authentication mechanisms.
Reply code 530 [SMTP-AUTH] is used for this purpose.
5. Recommended Actions
The MSA SHOULD do all of the following.
5.1. Enforce Address Syntax
An MSA SHOULD reject messages with illegal syntax in a sender or
recipient SMTP envelope address.
If the MSA examines or alters the message text in any way, except to
add trace header fields, it SHOULD reject messages with illegal
address syntax in address header fields.
Reply code 501 is to be used to reject a MAIL or RCPT command that
contains a detectably improper address.
When addresses are resolved after submission of the message body,
reply code 554 (with a suitable enhanced status code from
[SMTP-CODES]) is used after end-of-data, if the message contains
invalid addresses in the header.
5.2. Log Errors
The MSA SHOULD log message errors, especially apparent
misconfigurations of client software.
It can be very helpful to notify the administrator when problems are
detected with local mail clients. This is another advantage of
distinguishing submission from relay: system administrators might be
interested in local configuration problems, but not in client
problems at other sites.
Note that it is important to impose limits on such logging to prevent
certain forms of denial-of-service (DoS) attacks.
5.3. Apply Shorter Timeouts
The timeouts specified in Section 126.96.36.199 of RFC 5321 [SMTP-MTA] are
designed to deal with the many types of situations that can be
encountered on the public Internet. The relationship among clients
and servers corresponding to this specification is typically much
closer and more predictable. Submission clients behave differently
from relay client in some areas, especially tolerance for timeouts.
In practice, message submission clients tend to have short timeouts
(perhaps 2-5 minutes for a reply to any command). Submission servers
SHOULD respond to any command (even DATA) in fewer than 2 minutes.
When the submission server has a close administrative and/or network
relationship with the submission client(s) -- e.g., with a webmail
interface calling on a tightly bound submission server -- mutual
agreement on much shorter timeouts MAY be appropriate.
6. Optional Actions
The MSA MAY do any of the following.
6.1. Enforce Submission Rights
The MSA MAY issue an error response to a MAIL command if the address
in MAIL FROM appears to have insufficient submission rights or is not
authorized with the authentication used (if the session has been
Reply code 550 with an appropriate enhanced status code per
[SMTP-CODES], such as 5.7.1, is used for this purpose.
6.2. Enforce Permissions
The MSA MAY issue an error response to a RCPT command if inconsistent
with the permissions given to the user (if the session has been
Reply code 550 with an appropriate enhanced status code per
[SMTP-CODES], such as 5.7.1, is used for this purpose.
6.3. Check Message Data
The MSA MAY issue an error response to the DATA command or send a
failure result after end-of-data if the submitted message is
syntactically invalid, seems inconsistent with permissions given to
the user (if known), or violates site policy in some way.
Reply code 554 is used for syntactic problems in the data. Reply
code 501 is used if the command itself is not syntactically valid.
Reply code 550 with an appropriate enhanced status code per
[SMTP-CODES] (such as 5.7.1) is used to reject based on the
submitting user. Reply code 550 with an appropriate enhanced status
code (such as 5.7.0) is used if the message violates site policy.
6.4. Support for the Postmaster Address
If appropriate under local conditions and to facilitate conformance
with the "postmaster" requirements of [SMTP-MTA], the MSA MAY permit
a reduced degree of authentication for mail addressed to the
"postmaster" (or one of its alternate spelling forms, see
[SMTP-MTA]), in one or more domains, as compared to requirements
enforced for other addresses. Among other benefits, this provides an
address of last resort that can be used by authorized users to report
problems that otherwise prevent them from submitting mail.
6.5. Adjust Character Encodings
Subject to limits imposed by other protocols and specifications, the
MSA MAY convert among character sets or string encodings to improve
message usefulness, likelihood of delivery, or conformance with other
specifications or recommendations. Such conversions MAY include,
when necessary, replacement of addresses whose encoding does not
conform to RFC 5321 with ones that do, using information available
out of band.
7. Interaction with SMTP Extensions
The following table lists Standards Track and Experimental SMTP
extensions whose documents do not explicitly specify their
applicability to this protocol. Listed are the EHLO keyword, name,
an indication as to the use of the extension on the submit port, and
| Keyword | Name |Sub- | Reference |
| | |mission | |
|PIPELINING |Pipelining |SHOULD |[PIPELINING] |
|ENHANCEDSTATUSCODES |Enhanced Status Codes |SHOULD |[CODES-EXTENSION]|
|ETRN |Extended Turn |MUST NOT|[ETRN] |
| ... |Extended Codes |SHOULD |[SMTP-CODES] |
|DSN |Delivery Status |SHOULD |[DSN] |
| | Notification | | |
|SIZE |Message size |MAY |[SIZE] |
| ... |521 reply code |MUST NOT|[REPLY-521] |
|CHECKPOINT |Checkpoint/Restart |MAY |[CHECKPOINT] |
|BINARYMIME |Binary MIME |MAY |[CHUNKING] |
|CHUNKING |Chunking |MAY |[CHUNKING] |
|8BITMIME |Use 8-bit data |SHOULD |[RFC6152] |
|AUTH |Authentication |MUST |[SMTP-AUTH] |
|STARTTLS |Start TLS |MAY |[START-TLS] |
|NO-SOLICITING |Notification of |MAY |[RFC3865] |
| | no soliciting | | |
|MTRK |Message Tracking |MAY |[MSG-TRACK] |
|ATRN |On-Demand Relay |MUST NOT|[RFC2645] |
|DELIVERBY |Deliver By |MAY |[RFC2852] |
|CONPERM |Content Conversion |MAY |[RFC4141] |
| | Permission | | |
|CONNEG |Content Conversion |MAY |[RFC4141] |
| | Negotiation | | |
Future SMTP extensions SHOULD explicitly specify if they are valid on
the Submission port.
Some SMTP extensions are especially useful for message submission:
Extended Status Codes [SMTP-CODES] SHOULD be supported and used
according to [CODES-EXTENSION]. This permits the MSA to notify the
client of specific configuration or other problems in more detail
than the response codes listed in this memo. Because some rejections
are related to a site's security policy, care should be used not to
expose more detail to unauthenticated senders than is needed.
[PIPELINING] SHOULD be supported by the MSA.
[SMTP-AUTH] allows the MSA to validate the authority and determine
the identity of the submitting user and MUST be supported by the MSA.
[START-TLS] is the most widely used mechanism, at the time this
document was written, that allows the MUA and MSA to protect message
submission integrity and privacy.
Any references to the DATA command in this memo also refer to any
substitutes for DATA, such as the BDAT command used with [CHUNKING].
8. Message Modifications
Sites MAY modify submissions to ensure compliance with standards and
site policy. This section describes a number of such modifications
that are often considered useful.
NOTE: As a matter of guidance for local decisions to implement
message modification, a paramount rule is to limit such actions to
remedies for specific problems that have clear solutions. This is
especially true with address elements. For example, indiscriminately
appending a domain to an address or element that lacks one typically
results in more broken addresses. An unqualified address must be
verified to be a valid local part in the domain before the domain can
be safely added.
Any message forwarded or delivered by the MSA MUST conform to the
requirements of [SMTP-MTA] and [MESSAGE-FORMAT] or the requirements
permitted by extensions that are supported by the MSA and accepted by
the next-hop server.
Message modification can affect the validity of an existing message
signature, such as by DomainKeys Identified Mail (DKIM) [DKIM],
Pretty Good Privacy (PGP) [RFC4880], or Secure MIME (S/MIME)
[RFC5751], and can render the signature invalid. This, in turn, can
affect message handling by later receivers, such as filtering engines
that consider the presence or absence of a valid signature.
8.1. Add 'Sender'
The MSA MAY add or replace the 'Sender' field, if the identity of the
sender is known and this is not given in the 'From' field.
The MSA MUST ensure that any address it places in a 'Sender' field
is, in fact, a valid mail address.
8.2. Add 'Date'
The MSA MAY add a 'Date' field to the submitted message, if it lacks
it, or correct the 'Date' field if it does not conform to
8.3. Add 'Message-ID'
The MSA SHOULD add or replace the 'Message-ID' field, if it lacks it,
or it is not valid syntax (as defined by [MESSAGE-FORMAT]). Note
that a number of clients still do not generate 'Message-ID' fields.
8.4. Transfer Encode
The MSA MAY apply transfer encoding to the message according to MIME
conventions, if needed and not harmful to the MIME type.
8.5. Sign the Message
The MSA MAY (digitally) sign or otherwise add authentication
information to the message.
8.6. Encrypt the Message
The MSA MAY encrypt the message for transport to reflect
NOTE: To be useful, the addition of a signature and/or encryption by
the MSA generally implies that the connection between the MUA and MSA
must, itself, be secured in some other way, for example, by operating
inside of a secure environment, by securing the submission connection
at the transport layer, or by using an [SMTP-AUTH] mechanism that
provides for session integrity.
8.7. Resolve Aliases
The MSA MAY resolve and rewrite aliases (e.g., Canonical Name (CNAME)
records) for domain names, in the SMTP envelope and/or in address
fields of the header, subject to local policy.
NOTE: SMTP [SMTP-MTA] prohibits the use of domain name aliases in
addresses and the session-opening announcement. As with other SMTP
requirements, RFC 5321 effectively prohibits an MSA from forwarding
such messages into the public Internet. Nonetheless, unconditionally
resolving aliases could be harmful. For example, if www.example.net
and ftp.example.net are both aliases for mail.example.net, rewriting
them could lose useful information.
8.8. Header Rewriting
The MSA MAY rewrite local parts and/or domains in the SMTP envelope
and, optionally, in address fields of the header, according to local
policy. For example, a site may prefer to rewrite 'JRU' as
'J.Random.User' in order to hide login names and/or to rewrite
'squeaky.sales.example.net' as 'zyx.example.net' to hide machine
names and make it easier to move users.
However, only addresses, local-parts, or domains that match specific
local MSA configuration settings should be altered. It would be very
dangerous for the MSA to apply data-independent rewriting rules, such
as always deleting the first element of a domain name. So, for
example, a rule that strips the leftmost element of the domain, if
the complete domain matches '*.foo.example.net', would be acceptable.
The MSA MUST NOT rewrite a forward-pointing (destination) address in
a way that violates the constraints of [SMTP-MTA] on modifications of
local-parts. Changes to addressing and encoding, carried out in
conjunction with the action of Section 6.5, do not violate this
principle if the MSA has sufficient information available to
successfully and accurately apply the substitution.
9. Security Considerations
Separation of submission and relay of messages allows a site to
implement different policies for the two types of services, including
requiring the use of additional security mechanisms for one or both.
It can do this in a way that is simpler, both technically and
administratively. This increases the likelihood that policies will
be applied correctly.
Separation also can aid in tracking and preventing unsolicited bulk
For example, a site could configure its mail servers such that the
MSA requires authentication before accepting a message, and the MTA
rejects all RCPT commands for non-local users. This can be an
important element in a site's total email security policy.
If a site fails to require any form of authorization for message
submissions (see Section 3.3 for discussion), it is allowing open use
of its resources and name; unsolicited bulk email can be injected
using its facilities.
Section 3 includes further discussion of issues with some
Section 5.2 includes a cautionary note that unlimited logging can
enable certain forms of denial-of-service attacks.
10. IANA Considerations
The entries in Table 1 have been corrected (reference for NO-
SOLICITING) and extended (ATRN, DELIVERBY, CONPERM, and CONNEG). The
"SMTP Service Extensions" registry has been updated to reflect the
changed and new entries. Entries in the registry that do not appear
in the table above are correct and should not be altered.
The entry in the "SMTP Service Extensions" registry for RFC 4409 has
been updated to reference this document. The original reference for
Submit (RFC 2476), which should have been corrected earlier, has also
been updated to point to this document.
The entry in the "Service Name and Transport Protocol Port Number
Registry" for port 587 has been updated to point to this document.
The preparation and development of the current version of this
specification was stimulated by discussions in the IETF YAM and EAI
Working Groups. Dave Crocker, Subramanian Moonesamy, Barry Leiba,
John Levine, and others provided text that appeared in this document
or versions leading up to it.
Nathaniel Borenstein and Barry Leiba were instrumental in the
development of RFC 4409, the update to RFC 2476.
The original memo (RFC 2476) was developed, in part, based on
comments and discussions that took place on and off the IETF-Submit
mailing list. The help of those who took the time to review that
document and make suggestions is appreciated, especially that of Dave
Crocker, Ned Freed, Keith Moore, John Myers, and Chris Newman.
Special thanks to Harald Alvestrand, who got this effort started.
12.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SMTP-AUTH] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[SMTP-MTA] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
12.2. Informative References
[CHECKPOINT] Crocker, D. and N. Freed, "SMTP Service Extension for
Checkpoint/Restart", RFC 1845, September 1995.
[CHUNKING] Vaudreuil, G., "SMTP Service Extensions for Transmission
of Large and Binary MIME Messages", RFC 3030,
Freed, N., "SMTP Service Extension for Returning
Enhanced Error Codes", RFC 2034, October 1996.
[DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", RFC 6376,
[DSN] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.
[ETRN] De Winter, J., "SMTP Service Extension for Remote
Message Queue Starting", RFC 1985, August 1996.
[IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
Resnick, P., Ed., "Internet Message Format", RFC 5322,
[MSG-TRACK] Allman, E. and T. Hansen, "SMTP Service Extension for
Message Tracking", RFC 3885, September 2004.
[PIPELINING] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, September 2000.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version
3", STD 53, RFC 1939, May 1996.
[REPLY-521] Durand, A. and F. Dupont, "SMTP 521 Reply Code",
RFC 1846, September 1995.
[RFC2645] Gellens, R., "ON-DEMAND MAIL RELAY (ODMR) SMTP with
Dynamic IP Addresses", RFC 2645, August 1999.
[RFC2852] Newman, D., "Deliver By SMTP Service Extension",
RFC 2852, June 2000.
[RFC3865] Malamud, C., "A No Soliciting Simple Mail Transfer
Protocol (SMTP) Service Extension", RFC 3865,
[RFC4141] Toyoda, K. and D. Crocker, "SMTP and MIME Extensions for
Content Conversion", RFC 4141, November 2005.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and
R. Thayer, "OpenPGP Message Format", RFC 4880,
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
Service Extension for 8-bit MIME Transport", STD 71,
RFC 6152, March 2011.
[SIZE] Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10,
RFC 1870, November 1995.
[SMTP-CODES] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[START-TLS] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207, February 2002.
Appendix A. Major Changes from RFC 4409
The protocol specified by this document is not substantively
different from that of RFC 4409. However, the present specification
contains several clarifications and updates to reflect changes and
revisions to other documents subsequent to the publication of RFC
4409. The following specific changes may be of interest to some
o Updated several references to reflect more recent versions of the
various specifications. As part of this, reclassified RFC 4954 to
a normative reference (SMTP AUTH is a MUST for RFC 4409 and this
o Updated the text in Section 7 to reflect the existence and partial
population of the registry and the included table (Table 1) to
correct one entry and add others. See Section 10 for more
o Added new text (Section 5.3) to clarify that Submission Servers
should respond quickly.
o Added text to make it explicit that character encoding changes are
o Added text to make it clear that modifications to signed messages
may cause problems and that they should be carefully considered.
5775 Morehouse Drive
San Diego, CA 92121-2779
John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
Phone: +1 617 491 5735