rfc9422v1.txt   rfc9422.txt 
Internet Engineering Task Force (IETF) N. Freed Internet Engineering Task Force (IETF) N. Freed
Request for Comments: 9422 (deceased) Request for Comments: 9422
Category: Standards Track J. Klensin Category: Standards Track J. Klensin
ISSN: 2070-1721 January 2024 ISSN: 2070-1721 January 2024
The LIMITS SMTP Service Extension The LIMITS SMTP Service Extension
Abstract Abstract
This document defines a "Limits" extension for the Simple Mail This document defines a LIMITS extension for the Simple Mail Transfer
Transfer Protocol (SMTP), including submission, as well as the Local Protocol (SMTP), including submission, as well as the Local Mail
Mail Transfer Protocol (LMTP). It also defines an associated limit Transfer Protocol (LMTP). It also defines an associated limit
registry. The extension provides the means for an SMTP, submission, registry. The extension provides the means for an SMTP, submission,
or LMTP server to inform the client of limits the server intends to or LMTP server to inform the client of limits the server intends to
apply to the protocol during the current session. The client is then apply to the protocol during the current session. The client is then
able to adapt its behavior in order to conform to those limits. able to adapt its behavior in order to conform to those limits.
Status of This Memo Status of This Memo
This is an Internet Standards Track document. This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has (IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841. Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9422. https://www.rfc-editor.org/info/rfc9422.
Copyright Notice Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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in the Revised BSD License. in the Revised BSD License.
Table of Contents Table of Contents
1. Introduction 1. Introduction
2. Terminology 2. Terminology
3. The "Limits" SMTP Extension 3. The LIMITS SMTP Extension
3.1. Limits 3.1. Limits
3.2. Limit Naming Conventions 3.2. Limit Naming Conventions
3.3. Interaction with Pipelining 3.3. Interaction with Pipelining
3.4. Varying Limits 3.4. Varying Limits
3.5. Interaction with SMTP Minimums 3.5. Interaction with SMTP Minimums
3.6. Multiple EHLO Commands 3.6. Multiple EHLO Commands
3.7. Syntax Errors in the LIMITS Parameter Value 3.7. Syntax Errors in the LIMITS Parameter Value
3.8. Caching of Limit Settings between Sessions Involving the 3.8. Caching of Limit Settings between Sessions Involving the
Same Client and Server SMTP Same Client and Server SMTP
4. Initial Limits 4. Initial Limits
4.1. MAILMAX Limit 4.1. MAILMAX Limit
4.2. RCPTMAX Limit 4.2. RCPTMAX Limit
4.3. RCPTDOMAINMAX Limit 4.3. RCPTDOMAINMAX Limit
5. Example 5. Example
6. Security Considerations 6. Security Considerations
7. IANA Considerations 7. IANA Considerations
7.1. SMTP Service Extension Registry 7.1. SMTP Service Extension Registry
7.2. SMTP Server Limits Registry 7.2. SMTP Server Limits Registry
8. References 8. References
8.1. Normative References 8.1. Normative References
8.2. Informative References 8.2. Informative References
Acknowledgments Acknowledgments
Authors' Addresses Authors' Addresses
1. Introduction 1. Introduction
The Simple Mail Transfer Protocol provides the ability to transfer The Simple Mail Transfer Protocol provides the ability to transfer
[SMTP] or submit [SUBMIT] multiple email messages from one host to [SMTP] or submit [SUBMIT] multiple email messages from one host to
another, each with one or more recipients, using a single or multiple another, each with one or more recipients, using a single or multiple
connections. connections.
The Local Mail Transfer Protocol [LMTP] provides the ability to The Local Mail Transfer Protocol [LMTP] provides the ability to
deliver messages to a system without its own mail queues. Like SMTP, deliver messages to a system without its own mail queues. Like SMTP,
it allows multiple messages with multiple recipients. it allows multiple messages with multiple recipients.
In order to conserve resources as well as protect themselves from In order to conserve resources as well as protect themselves from
malicious clients, it is necessary for servers to enforce limits on malicious clients, it is necessary for servers to enforce limits on
various aspects of the protocol, e.g., a limit on the number of various aspects of the protocol, e.g., a limit on the number of
recipients that can be specified in a single transaction. recipients that can be specified in a single transaction.
Additionally, servers may also wish to alter the limits they apply Additionally, servers may also wish to alter the limits they apply
depending on their assessment of the reputation of a particular depending on their assessment of the reputation of a particular
client. client.
The variability of the limits that may be in effect creates a The variability of the limits that may be in effect creates a
situation where clients may inadvertently exceed a particular situation where clients may inadvertently exceed a particular
server's limits, causing servers to respond with temporary (or in server's limits, causing servers to respond with temporary (or in
some cases, permanent) errors. This in turn can lead to delays or some cases, permanent) errors. This in turn can lead to delays or
even failures in message transfer. even failures in message transfer.
The "Limits" extension provides the means for a server to inform a The LIMITS extension provides the means for a server to inform a
client about specific limits in effect for a particular SMTP or LMTP client about specific limits in effect for a particular SMTP or LMTP
session in the EHLO or LHLO command response. This information, session in the EHLO or LHLO command response. This information,
combined with the inherent flexibility of these protocols, makes it combined with the inherent flexibility of these protocols, makes it
possible for clients to avoid server errors and the problems they possible for clients to avoid server errors and the problems they
cause. cause.
SMTP and LMTP servers have always been able to announce a limit using SMTP and LMTP servers have always been able to announce a limit using
distinguished syntax in a reply, but this approach requires that the distinguished syntax in a reply, but this approach requires that the
client first needs to issue a command. The mechanism specified here client first needs to issue a command. The mechanism specified here
avoids the overhead of that approach by announcing limits prior to avoids the overhead of that approach by announcing limits prior to
any substantive interaction. any substantive interaction.
Limits are registered with the IANA. Each registration includes the Limits are registered with the IANA. Each registration includes the
limit name, value syntax, and a description of its semantics. limit name, value syntax, and a description of its semantics.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This specification uses the Augmented Backus-Naur Form [ABNF] This specification uses the Augmented Backus-Naur Form [ABNF]
notation and its core rules to define the formal syntax of the notation and its core rules to define the formal syntax of the LIMITS
"Limits" extension. extension.
This specification makes extensive use of the terminology specified This specification makes extensive use of the terminology specified
and used in [SMTP]. and used in [SMTP].
3. The "Limits" SMTP Extension 3. The LIMITS SMTP Extension
Extensions to SMTP are defined in Section 2.2 of SMTP [SMTP]. [LMTP] Extensions to SMTP are defined in Section 2.2 of SMTP [SMTP]. LMTP
inherits SMTP's extension mechanism. [LMTP] inherits SMTP's extension mechanism.
The name of the extension is "Limits". Servers implementing this The name of the extension is LIMITS. Servers implementing this
extension advertise an additional "LIMITS" EHLO (LHLO in LMTP) extension advertise an additional LIMITS EHLO (LHLO in LMTP) keyword.
keyword. The associated parameter is used by the server to The associated parameter is used by the server to communicate one or
communicate one or more limits, each with an optional value, to the more limits, each with an optional value, to the client. The syntax
client. The syntax of the parameter is: of the parameter is:
limits-param = limit-name-value 0*[SP limit-name-value] limits-param = limit-name-value 0*[SP limit-name-value]
limit-name-value = limit-name ["=" limit-value] limit-name-value = limit-name ["=" limit-value]
limit-name = 1*(ALPHA / DIGIT / "-" / "_") limit-name = 1*(ALPHA / DIGIT / "-" / "_")
limit-value = 1*(%x21-3A / %x3C-7E) ; Any VCHAR except ";" limit-value = 1*(%x21-3A / %x3C-7E) ; Any VCHAR except ";"
This extension introduces no new SMTP commands and does not alter any This extension introduces no new SMTP commands and does not alter any
existing command. However, it is possible for a LIMITS parameter to existing command. However, it is possible for a LIMITS parameter to
be associated with another SMTP extension that does these things. be associated with another SMTP extension that does these things.
3.1. Limits 3.1. Limits
In order to achieve consistent behavior, all limits MUST be In order to achieve consistent behavior, all limits MUST be
registered with the IANA, as described below. registered with the IANA, as described below.
3.2. Limit Naming Conventions 3.2. Limit Naming Conventions
Limit names MUST be comprehensible, but also should be kept as short Limit names MUST be comprehensible, but also should be kept as short
as possible. The use of commonly understood abbreviations, e.g., as possible. The use of commonly understood abbreviations, e.g.,
"MAX" for "maximum", is encouraged. "MAX" for "maximum", is encouraged.
When a limit is associated with a particular command, its name SHOULD When a limit is associated with a particular command, its name SHOULD
begin with the name of that command. begin with the name of that command.
Limit names SHOULD end with one or more terms that describe the type Limit names SHOULD end with one or more terms that describe the type
of limit. of limit.
3.3. Interaction with Pipelining 3.3. Interaction with Pipelining
The "Pipelining" extension [PIPELINING] is commonly used to improve The "Pipelining" extension [PIPELINING] is commonly used to improve
performance, especially over high latency connections. Pipelining performance, especially over high latency connections. Pipelining
allows an entire transaction to be sent without checking responses, allows an entire transaction to be sent without checking responses,
and in some cases it may be possible to send multiple transactions. and in some cases it may be possible to send multiple transactions.
The use of pipelining affects limits in an important way: Since a The use of pipelining affects limits in an important way: Since a
pipelining client cannot check intermediate command responses without pipelining client cannot check intermediate command responses without
stalling the pipeline, it cannot count the number of successful stalling the pipeline, it cannot count the number of successful
versus failed responses and adjust its behavior accordingly. Limit versus failed responses and adjust its behavior accordingly. Limit
designers need to take this into account. designers need to take this into account.
For example, it may seem like it would be better to impose a limit on For example, it may seem like it would be better to impose a limit on
the number of successful RCPT TO commands as opposed to the way the the number of successful RCPT TO commands as opposed to the way the
RCPTMAX limit is specified in Section 4.2 below. But counting the RCPTMAX limit is specified in Section 4.2 below. But counting the
total number of RCPT TOs is simple, whereas counting the number of total number of RCPT TOs is simple, whereas counting the number of
successful RCPT TO stalls the pipeline. successful RCPT TO stalls the pipeline.
3.4. Varying Limits 3.4. Varying Limits
This extension provides an announcement as part of the reply to an This extension provides an announcement as part of the reply to an
EHLO command. EHLO command.
Some servers vary their limits, as a session progresses, based on Some servers vary their limits, as a session progresses, based on
their obtaining more information. This extension does not attempt to their obtaining more information. This extension does not attempt to
handle in-session limit changes. handle in-session limit changes.
3.5. Interaction with SMTP Minimums 3.5. Interaction with SMTP Minimums
Section 4.5.3.1 of SMTP [SMTP] specifies minimum values for various Section 4.5.3.1 of SMTP [SMTP] specifies minimum values for various
server sizes, limits, and timeouts, e.g., servers must accept a server sizes, limits, and timeouts, e.g., servers must accept a
minimum of 100 RCPT TO commands (Section 4.5.3.1.8 of SMTP [SMTP]). minimum of 100 RCPT TO commands (Section 4.5.3.1.8 of SMTP [SMTP]).
Unfortunately, the reality is that servers routinely impose smaller Unfortunately, the reality is that servers routinely impose smaller
limits than what SMTP requires, and when this is done it is limits than what SMTP requires, and when this is done it is
especially important for clients to be aware that this is happening. especially important for clients to be aware that this is happening.
For this reason there is no requirement that the limits advertised by For this reason there is no requirement that the limits advertised by
this extension comply with the minimums imposed by SMTP. this extension comply with the minimums imposed by SMTP.
3.6. Multiple EHLO Commands 3.6. Multiple EHLO Commands
These protocols require that the EHLO command (LHLO in LMTP) be These protocols require that the EHLO command (LHLO in LMTP) be
reissued under certain circumstances, e.g., after successful reissued under certain circumstances, e.g., after successful
authentication [AUTH] or negotiation of a security layer [STARTTLS]. authentication [AUTH] or negotiation of a security layer [STARTTLS].
Servers MAY return updated limits any time the protocol requires Servers MAY return updated limits any time the protocol requires
clients to reissue the EHLO command. clients to reissue the EHLO command.
Clients MUST discard any previous limits in favor of those provided Clients MUST discard any previous limits in favor of those provided
by the most recent EHLO. This includes the case where the original by the most recent EHLO. This includes the case where the original
EHLO provided a set of limits but the subsequent EHLO did not; in EHLO provided a set of limits but the subsequent EHLO did not; in
this case, the client MUST act as if no limits were communicated. this case, the client MUST act as if no limits were communicated.
3.7. Syntax Errors in the LIMITS Parameter Value 3.7. Syntax Errors in the LIMITS Parameter Value
Syntax errors in the basic parameter syntax are best handled by Syntax errors in the basic parameter syntax are best handled by
ignoring the value in its entirety; in this case, clients SHOULD ignoring the value in its entirety; in this case, clients SHOULD
proceed as if the LIMITS extension had not been used. proceed as if the LIMITS extension had not been used.
Syntax or other errors in the value syntax of a specific limit, Syntax or other errors in the value syntax of a specific limit,
including unrecognized value keywords, are best handled by ignoring including unrecognized value keywords, are best handled by ignoring
that limit; in this case, the client MUST proceed as if that limit that limit; in this case, the client MUST proceed as if that limit
had not been specified. had not been specified.
It is possible that a future specification may create multiple limits It is possible that a future specification may create multiple limits
that are interrelated in some way; in this case, that specification that are interrelated in some way; in this case, that specification
MUST specify how an error in the value syntax of one limit affects MUST specify how an error in the value syntax of one limit affects
the other limits. the other limits.
3.8. Caching of Limit Settings between Sessions Involving the Same 3.8. Caching of Limit Settings between Sessions Involving the Same
Client and Server SMTP Client and Server SMTP
Clients MAY cache limits determined during one session and use them Clients MAY cache limits determined during one session and use them
to optimize their behavior for subsequent sessions. However, since to optimize their behavior for subsequent sessions. However, since
servers are free to adjust their limits at any time, clients MUST be servers are free to adjust their limits at any time, clients MUST be
able to accommodate any limit changes that occur between sessions. able to accommodate any limit changes that occur between sessions.
4. Initial Limits 4. Initial Limits
An initial set of limits is specified in the following sections. An initial set of limits is specified in the following sections.
4.1. MAILMAX Limit 4.1. MAILMAX Limit
Name: MAILMAX Name: MAILMAX
Value syntax: %x31-39 0*5DIGIT ; 0 not allowed, 6-digit maximum Value syntax: %x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum
Description: MAILMAX specifies the maximum number of transactions Description: MAILMAX specifies the maximum number of transactions
(MAIL FROM commands) the server will accept in a single session. (MAIL FROM commands) the server will accept in a single session.
The count includes all MAIL FROM commands, regardless of whether The count includes all MAIL FROM commands, regardless of whether
they succeed or fail. they succeed or fail.
Restrictions: None. Restrictions: None.
Security Considerations: See Section 6 Security Considerations: See Section 6
4.2. RCPTMAX Limit 4.2. RCPTMAX Limit
Name: RCPTMAX Name: RCPTMAX
Value syntax: %x31-39 0*5DIGIT ; 0 not allowed, 6-digit maximum Value syntax:
%x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum
Description: RCPTMAX specifies the maximum number of RCPT TO Description: RCPTMAX specifies the maximum number of RCPT TO
commands the server will accept in a single transaction. It is commands the server will accept in a single transaction. It is
not a limit on the actual number of recipients the message ends up not a limit on the actual number of recipients the message ends up
being sent to; a single RCPT TO command may produce multiple being sent to; a single RCPT TO command may produce multiple
recipients or, in the event of an error, none. recipients or, in the event of an error, none.
Restrictions: None. Restrictions: None.
Security Considerations: See Section 6 Security Considerations: See Section 6
4.3. RCPTDOMAINMAX Limit 4.3. RCPTDOMAINMAX Limit
Name: RCPTDOMAINMAX Name: RCPTDOMAINMAX
Value syntax: %x31-39 0*5DIGIT ; 0 not allowed, 6-digit maximum Value syntax: %x31-39 0*5DIGIT ; "0" not allowed, six-digit maximum
Description: RCPTDOMAINMAX specifies the maximum number of different Description: RCPTDOMAINMAX specifies the maximum number of different
domains that can appear in a recipient (RCPT TO) address within a domains that can appear in a recipient (RCPT TO) address within a
single session. This limit is imposed by some servers that bind single session. This limit is imposed by some servers that bind
to a specific internal delivery mechanism on receipt of the first to a specific internal delivery mechanism on receipt of the first
RCPT TO command. RCPT TO command.
Restrictions: None. Restrictions: None.
Security Considerations: See Section 6 Security Considerations: See Section 6
5. Example 5. Example
A server announces two limits it implements to the client, along with A server announces two limits it implements to the client, along with
various other supported extensions, as follows: various other supported extensions, as follows:
S: [wait for open connection] S: [wait for open connection]
C: [open connection to server] C: [open connection to server]
S: 220 mail.example.com ESMTP service ready S: 220 mail.example.com ESMTP service ready
C: EHLO example.org C: EHLO example.org
S: 250-mail.example.com S: 250-mail.example.com
S: 250-SMTPUTF8 S: 250-SMTPUTF8
S: 250-LIMITS RCPTMAX=20 MAILMAX=5 S: 250-LIMITS RCPTMAX=20 MAILMAX=5
S: 250-SIZE 100000000 S: 250-SIZE 100000000
S: 250-8BITMIME S: 250-8BITMIME
S: 250-ENHANCEDSTATUSCODES S: 250-ENHANCEDSTATUSCODES
S: 250-PIPELINING S: 250-PIPELINING
S: 250-CHUNKING S: 250-CHUNKING
S: 250 STARTTLS S: 250 STARTTLS
The client now knows to limit the number of recipients in a The client now knows to limit the number of recipients in a
transaction to twenty and the number of transactions in a session to transaction to twenty and the number of transactions in a session to
five. five.
6. Security Considerations 6. Security Considerations
A malicious server can use limits to overly constrain client A malicious server can use limits to overly constrain client
behavior, causing excessive use of client resources. behavior, causing excessive use of client resources.
A malicious client may use the limits a server advertises to optimize A malicious client may use the limits a server advertises to optimize
the delivery of unwanted messages. the delivery of unwanted messages.
A man-in-the-middle attack on unprotected SMTP connections can be A man-in-the-middle attack on unprotected SMTP connections can be
used to cause clients to misbehave, which in turn could result in used to cause clients to misbehave, which in turn could result in
delivery delays or failures. Loss of reputation for the client could delivery delays or failures. Loss of reputation for the client could
also occur. also occur.
All that said, decades of operational experience with the SMTP "SIZE" All that said, decades of operational experience with the SMTP "SIZE"
extension [SIZE], which provides servers with the ability to indicate extension [SIZE], which provides servers with the ability to indicate
message size, indicates that such abuse is rare and unlikely to be a message size, indicates that such abuse is rare and unlikely to be a
significant problem. significant problem.
Use of the Limits extension to provide client-specific information - Use of the LIMITS extension to provide client-specific information -
as opposed to general server limits - unavoidably provides senders as opposed to general server limits - unavoidably provides senders
with feedback about their reputation. Malicious senders can exploit with feedback about their reputation. Malicious senders can exploit
this in various ways, e.g., start by sending good email and then, this in various ways, e.g., start by sending good email and then,
once their reputation is established, sending bad email. once their reputation is established, sending bad email.
7. IANA Considerations 7. IANA Considerations
7.1. SMTP Service Extension Registry 7.1. SMTP Service Extension Registry
The IANA has added "LIMITS" to the "SMTP Service Extensions" The IANA has added "LIMITS" to the "SMTP Service Extensions"
registry: registry:
EHLO Keyword: LIMITS EHLO Keyword: LIMITS
Description: Server limits Description: Server limits
Reference: RFC 9422 Reference: RFC 9422
Note: See "SMTP Server Limits" registry. Note: See "SMTP Server Limits" registry.
7.2. SMTP Server Limits Registry 7.2. SMTP Server Limits Registry
The IANA has created a new registry in the Mail Parameters group for The IANA has created a new registry in the Mail Parameters group for
SMTP server limits. The policy for this registry is "Specification SMTP server limits. The policy for this registry is "Specification
Required". Registry entries consist of these required values: Required". Registry entries consist of these required values:
1. The name of the limit. 1. The name of the limit.
2. The syntax of the limit value, if the limit has one. This syntax 2. The syntax of the limit value, if the limit has one. This syntax
MUST conform to the provisions of Section 3 above. In most MUST conform to the provisions of Section 3 above. In most
cases, the syntax will consist of a keyword designating the limit cases, the syntax will consist of a keyword designating the limit
type followed by a limit value, using a "name=value" form as type followed by a limit value, using a "name=value" form as
illustrated by the registrations created by this specification in illustrated by the registrations created by this specification in
Section 4 above. Use of ABNF [ABNF] is preferred but not Section 4 above. Use of ABNF [ABNF] is preferred but not
required. If there is no limit value, that should be explicit in required. If there is no limit value, that should be explicit in
the registration request and the registry. the registration request and the registry.
3. A description of the limit's semantics. 3. A description of the limit's semantics.
4. Restrictions, if any, on the use of the limit. If the limit is 4. Restrictions, if any, on the use of the limit. If the limit is
specific to any of SMTP, message submission, or LMTP, it should specific to any of SMTP, message submission, or LMTP, it should
be documented here. be documented here.
5. Security considerations for the limit. 5. Security considerations for the limit.
The Designated Expert(s) appointed under the "Specification Required" The Designated Expert(s) appointed under the "Specification Required"
procedure should check that registration requests are consistent with procedure should check that registration requests are consistent with
the requirements and intent of the extension mechanisms associated the requirements and intent of the extension mechanisms associated
with SMTP [SMTP], Section 3 above, and the provision of this with SMTP [SMTP], Section 3 above, and the provision of this
specification more generally and offer advice accordingly. specification more generally and offer advice accordingly.
The IANA has registered the limits specified in Section 4 of this The IANA has registered the limits specified in Section 4 of this
document. document.
8. References 8. References
8.1. Normative References 8.1. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[PIPELINING] [PIPELINING]
Freed, N., "SMTP Service Extension for Command Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920, Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
September 2000, <https://www.rfc-editor.org/info/rfc2920>. September 2000, <https://www.rfc-editor.org/info/rfc2920>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>. <https://www.rfc-editor.org/info/rfc5321>.
8.2. Informative References 8.2. Informative References
[AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service [AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, Extension for Authentication", RFC 4954,
DOI 10.17487/RFC4954, July 2007, DOI 10.17487/RFC4954, July 2007,
<https://www.rfc-editor.org/info/rfc4954>. <https://www.rfc-editor.org/info/rfc4954>.
[LMTP] Myers, J., "Local Mail Transfer Protocol", RFC 2033, [LMTP] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
DOI 10.17487/RFC2033, October 1996, DOI 10.17487/RFC2033, October 1996,
<https://www.rfc-editor.org/info/rfc2033>. <https://www.rfc-editor.org/info/rfc2033>.
[SIZE] Klensin, J., Freed, N., and K. Moore, "SMTP Service [SIZE] Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10, RFC 1870, Extension for Message Size Declaration", STD 10, RFC 1870,
DOI 10.17487/RFC1870, November 1995, DOI 10.17487/RFC1870, November 1995,
<https://www.rfc-editor.org/info/rfc1870>. <https://www.rfc-editor.org/info/rfc1870>.
[STARTTLS] Hoffman, P., "SMTP Service Extension for Secure SMTP over [STARTTLS] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <https://www.rfc-editor.org/info/rfc3207>. February 2002, <https://www.rfc-editor.org/info/rfc3207>.
[SUBMIT] Gellens, R. and J. Klensin, "Message Submission for Mail", [SUBMIT] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<https://www.rfc-editor.org/info/rfc6409>. <https://www.rfc-editor.org/info/rfc6409>.
Acknowledgments Acknowledgments
The concept for this extension came from, and was developed by, Ned The concept for this extension came from, and was developed by, Ned
Freed and most of this specification, including substantially of the Freed and most of this specification, including substantially of the
technical details, was written by him. Unfortunately, he became ill technical details, was written by him. Unfortunately, he became ill
and eventually passed away in May 2022 without being able to complete and eventually passed away in May 2022 without being able to complete
the document and manage it through IETF Last Call. His contributions the document and manage it through IETF Last Call. His contributions
to the Internet, work in the IETF, and the personal style that made to the Internet, work in the IETF, and the personal style that made
both possible are irreplaceable and greatly missed. With the support both possible are irreplaceable and greatly missed. With the support
of the community, John Klensin picked the document up, responded to of the community, John Klensin picked the document up, responded to
some additional feedback, and got the document into what is believed some additional feedback, and got the document into what is believed
to be a finished state. In the interest of preserving this as Ned's to be a finished state. In the interest of preserving this as Ned's
document, a few comments that proposed additional features and document, a few comments that proposed additional features and
options were set aside for future work rather than our having to options were set aside for future work rather than our having to
guess at whether Ned would have approved of them. guess at whether Ned would have approved of them.
The acknowledgments below are divided into two parts, those written The acknowledgments below are divided into two parts, those written
by Ned and those associated with input to the document after his by Ned and those associated with input to the document after his
passing. passing.
* Acknowledgments from the Last Version Prepared by Ned Freed * Acknowledgments from the Last Version Prepared by Ned Freed
A lot of people have helped make this specification possible. The A lot of people have helped make this specification possible. The
author wishes to thank Claus Assmann, Laura Atkins, Alex Brotman, author wishes to thank Claus Assmann, Laura Atkins, Alex Brotman,
Richard Clayton, Dave Crocker, Viktor Dukhovni, Arnt Gulbrandsen, Richard Clayton, Dave Crocker, Viktor Dukhovni, Arnt Gulbrandsen,
Jeremy Harris, Todd Herr, Mike Hillyer, Matthias Leisi, John Jeremy Harris, Todd Herr, Mike Hillyer, Matthias Leisi, John
Klensin, Valdis Klētnieks, John Levine, Alexey Melnikov, Keith Klensin, Valdis Klētnieks, John Levine, Alexey Melnikov, Keith
Moore, Michael Peddemors, Hector Santos, George Schlossnagle, Rolf Moore, Michael Peddemors, Hector Santos, George Schlossnagle, Rolf
E. Sonneveld, and Alessandro Vesely for their contributions and E. Sonneveld, and Alessandro Vesely for their contributions and
reviews. reviews.
* Acknowledgments from Versions Subsequent to Ned Freed's Passing * Acknowledgments from Versions Subsequent to Ned Freed's Passing
Additional helpful suggestions were received when the draft was Additional helpful suggestions were received when the draft was
requeued in the last part of 2022 and thereafter. Reviews and requeued in the last part of 2022 and thereafter. Reviews and
suggestions from Alex Brotman, Dave Crocker, Gene Hightower, suggestions from Alex Brotman, Dave Crocker, Gene Hightower,
Murray S. Kucherawy, Barry Leiba, John Levine, and Phil Pennock Murray S. Kucherawy, Barry Leiba, John Levine, and Phil Pennock
were especially helpful in identifying errors and suggesting were especially helpful in identifying errors and suggesting
clarifying some issues with the document without changing Ned's clarifying some issues with the document without changing Ned's
fundamental issues or very much of his text. fundamental issues or very much of his text.
IETF Last Call comments (and comments immediately before it IETF Last Call comments (and comments immediately before it
started) from people not listed above that resulted in document started) from people not listed above that resulted in document
changes were received from Amanda Baber (for IANA), Linda Dunbar, changes were received from Amanda Baber (for IANA), Linda Dunbar,
Paul Kyzivat, and Phil Pennock. Paul Kyzivat, and Phil Pennock.
Authors' Addresses Authors' Addresses
Ned Freed Ned Freed
(deceased)
John C. Klensin John C. Klensin
1770 Massachusetts Ave, Suite 322 1770 Massachusetts Ave, Suite 322
Cambridge, MA 02140 Cambridge, MA 02140
United States of America United States of America
Email: john-ietf@jck.com Email: john-ietf@jck.com
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