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rfc:rfc6710

Internet Engineering Task Force (IETF) A. Melnikov Request for Comments: 6710 Isode Ltd Category: Standards Track K. Carlberg ISSN: 2070-1721 G11

                                                           August 2012

Simple Mail Transfer Protocol Extension for Message Transfer Priorities

Abstract

 This memo defines an extension to the SMTP (Simple Mail Transfer
 Protocol) service whereby messages are given a label to indicate
 preferential handling, to enable mail handling nodes to take this
 information into account for onward processing.

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
 http://www.rfc-editor.org/info/rfc6710.

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Melnikov & Carlberg Standards Track [Page 1] RFC 6710 Message Transfer Priority SMTP Extension August 2012

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
 3.  Definition of the Priority SMTP Extension  . . . . . . . . . .  4
 4.  Handling of Messages Received via SMTP . . . . . . . . . . . .  5
   4.1.  Handling of the MT-PRIORITY Parameter by the Receiving
         SMTP Server  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.2.  Relay of Messages to Other Conforming SMTP/LMTP Servers  .  6
   4.3.  Relay of Messages to Non-Conforming SMTP/LMTP Servers  . .  7
   4.4.  Mailing Lists and Aliases  . . . . . . . . . . . . . . . .  7
   4.5.  Gatewaying a Message into a Foreign Environment  . . . . .  7
   4.6.  Interaction with the DSN SMTP Extension  . . . . . . . . .  7
 5.  The Priority Service Extension . . . . . . . . . . . . . . . .  8
   5.1.  Expedited Transfer . . . . . . . . . . . . . . . . . . . .  9
   5.2.  Timely Delivery  . . . . . . . . . . . . . . . . . . . . . 10
 6.  Use of MT-PRIORITY with LMTP . . . . . . . . . . . . . . . . . 10
 7.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 8.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
 9.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 14
   9.1.  Multiple MX Records  . . . . . . . . . . . . . . . . . . . 14
   9.2.  Priority Assignment Policies . . . . . . . . . . . . . . . 15
 10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10.1. Requirements on Priority Assignment Policy
         Registrations  . . . . . . . . . . . . . . . . . . . . . . 17
   10.2. Initial Priority Assignment Policy Registrations . . . . . 18
 11. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
   12.2. Informative References . . . . . . . . . . . . . . . . . . 20
 Appendix A.  Priority Assignment Policy for Military Messaging . . 22
 Appendix B.  Priority Assignment Policy for MIXER  . . . . . . . . 23
 Appendix C.  Priority Assignment Policy for National Security
              / Emergency Preparedness (NS/EP)  . . . . . . . . . . 24
 Appendix D.  Possible Implementation Strategies  . . . . . . . . . 25
   D.1.  Probability  . . . . . . . . . . . . . . . . . . . . . . . 25
   D.2.  Preemption of Sessions or Transactions . . . . . . . . . . 25
   D.3.  Resource Allocation Models . . . . . . . . . . . . . . . . 26
 Appendix E.  Background on Design Choices  . . . . . . . . . . . . 26
 Appendix F.  Acknowledgements  . . . . . . . . . . . . . . . . . . 27

Melnikov & Carlberg Standards Track [Page 2] RFC 6710 Message Transfer Priority SMTP Extension August 2012

1. Introduction

 Where resources for switching or transferring messages are
 constrained (e.g., bandwidth, round trip time, transition storage, or
 processing capability), it is desirable to give preferential handling
 to some messages over others, according to their labeled priority.
 This is particularly important during emergencies for first
 responders (Appendix C) and for environments such as military
 (Appendix A) and aviation (Appendix B) messaging, where messages have
 high operational significance, and the consequences of extraneous
 delay can be significant.
 In order for an SMTP receiver to be able to relay higher-priority
 messages first, there needs to be a mechanism to communicate (during
 both Message Submission [RFC6409] and Message Transfer [RFC5321]) the
 priority of each message.  This specification defines this mechanism
 by specification of an SMTP [RFC5321] extension.
 In order to permit end-to-end use of this extension across an email
 infrastructure that does not support it, a companion tunneling
 mechanism is defined in [PRIORITY-TUNNELING] that uses a new message
 header field [RFC5322].
 This extension provides services to some classes of users in networks
 with limited available bandwidth or long round trip times, when the
 actual message transfer over the network can create a significant
 portion of the overall message delivery time from a sender to a
 recipient, for example, over a satellite or high-frequency radio
 link.  It is also useful in case of a Mail Transfer Agent (MTA) queue
 buildup due to the rate of incoming messages being higher than the
 rate of outgoing messages.  When neither of the two conditions
 mentioned above is true, the use of the MT-PRIORITY SMTP extension
 will not result in better SMTP service to any user.  Also note that
 while this SMTP extension can help in improving delivery speed for
 higher-priority messages, it does not provide any guarantees that for
 two given messages with priorities M and N (M > N) submitted
 simultaneously, the message with priority M will arrive earlier than
 the message with priority N. That is, this extension calls for best
 effort to provide preferential processing.
 Besides the actions taken at the application level, it can thus be
 important to deploy priority or precedence mechanisms offered by the
 network itself to ensure timely delivery of the emails.  Examples
 would be the use of DiffServ [RFC2474], RSVP [RFC2205], and [RFC6401]
 (an extension to RSVP that prioritizes reservations).

Melnikov & Carlberg Standards Track [Page 3] RFC 6710 Message Transfer Priority SMTP Extension August 2012

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119] when they
 appear in ALL CAPS.  These words also appear in this document in
 lower case as plain English words, absent their normative meanings.
 The formal syntax uses the Augmented Backus-Naur Form (ABNF)
 [RFC5234] notation including the core rules defined in Appendix B of
 RFC 5234 [RFC5234].
 In examples, "C:" and "S:" indicate lines sent by the client and
 server, respectively.  Line breaks that do not start with a new "C:"
 or "S:" exist for editorial reasons and are not a part of the
 protocol.
 This document uses the term "priority" specifically in relation to
 the internal treatment of a message by the server.  Messages with
 higher priorities may be given expedited handling, and those with
 lower priorities may be handled only as resources become available.

3. Definition of the Priority SMTP Extension

 The Priority SMTP service extension is defined as follows:
 1.  The textual name of this extension is "Priority Message
     Handling".
 2.  The EHLO keyword value associated with this extension is
     "MT-PRIORITY".
 3.  The EHLO keyword has an OPTIONAL parameter that conveys the name
     of the Priority Assignment Policy (see Section 9.2) used by the
     server.  (See the <mt-priority-ehlo> ABNF non-terminal in
     Section 7 for details of its syntax.)  Absence of the parameter
     means that the server is unwilling to disclose its Priority
     Assignment Policy.  Clients can choose to use the MT-PRIORITY
     SMTP extension even if they don't recognize a particular Priority
     Assignment Policy name advertised by a server.
 4.  No additional SMTP verbs are defined by this extension.
 5.  One optional parameter ("MT-PRIORITY") is added to the MAIL FROM
     command.  The value associated with this parameter is a decimal
     integer number from -9 to 9 (inclusive) indicating the priority
     of the email message (see Appendix E for more details on why this
     range was selected).  The syntax of the MT-PRIORITY parameter is

Melnikov & Carlberg Standards Track [Page 4] RFC 6710 Message Transfer Priority SMTP Extension August 2012

     described by the <priority-value> ABNF non-terminal defined in
     Section 7.  Higher numbers mean higher priority.
 6.  The maximum length of a MAIL FROM command line is increased by 15
     octets by the possible addition of a space, the MT-PRIORITY
     keyword, and a priority value.
 7.  The MT-PRIORITY extension is valid for the submission service
     [RFC6409] and the Local Mail Transfer Protocol (LMTP) [RFC2033].

4. Handling of Messages Received via SMTP

 This section describes how a conforming SMTP server should handle any
 messages received via SMTP.

4.1. Handling of the MT-PRIORITY Parameter by the Receiving SMTP Server

 The following rules apply to SMTP transactions in a server that
 supports the MT-PRIORITY parameter:
 1.  If any of the associated <esmtp-value>s (as defined in Section
     4.1.2 of [RFC5321]) are not syntactically valid, or if there is
     more than one MT-PRIORITY parameter in a particular MAIL FROM
     command, the server MUST return an error, for example "501 syntax
     error in parameter" (with the 5.5.2 Enhanced Status Code
     [RFC2034] [RFC5248]).
 2.  When inserting a Received header field as specified in Section
     4.4 of [RFC5321], the compliant MTA/MSA (Mail Submission Agent)
     SHOULD include the "PRIORITY" clause whose syntax is specified in
     Section 7.
 3.  The received MT-PRIORITY parameter value SHOULD be logged as part
     of any logging of message transactions.
 4.  If the sending SMTP client specified the MT-PRIORITY parameter to
     the MAIL FROM command, then the value of this parameter is the
     message priority.
 5.  If no priority has been determined by the above, the server may
     use its normal policies to set the message's priority.  By
     default, each message has priority 0.
 The SMTP server MUST NOT allow "upgraded" (positive) priorities from
 untrusted (e.g., unauthenticated) or unauthorized sources.  (One
 example of an "unauthorized source" might be an SMTP sender that
 successfully authenticated using SMTP AUTH, but that is not
 explicitly authorized to use the SMTP MT-PRIORITY service.  In case

Melnikov & Carlberg Standards Track [Page 5] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 of MTA-to-MTA transfer, such authorization will usually be done as a
 bilateral agreement between two domains to honor priorities from each
 other.)  The server MAY, however, allow an untrusted source to lower
 its own message's priorities -- consider, for example, an email
 marketer that voluntarily sends its marketing messages at a negative
 priority.
 The SMTP server MAY also alter the message priority (to lower or to
 raise it) in order to enforce some other site policy.  (Note that
 this also includes the case in which the priority is not explicitly
 specified.)  For example, an MSA might have a mapping table that
 assigns priorities to messages based on authentication credentials.
 If the SMTP server changes (lowers or raises) the priority of a
 message, it SHOULD use the X.3.6 Enhanced Status Code [RFC2034] in
 its response to the MAIL FROM or in the final response to the DATA
 (or similar) command.  The human readable text part after the status
 code contains the new priority, followed by SP (ASCII space) and
 explanatory human readable text.
 Alternatively, an SMTP server that is an MSA MAY reject a message
 based on the determined priority.  In such cases, the MSA SHOULD use
 the 450 or 550 reply code.  The corresponding Enhanced Status Code
 MUST be X.7.15 [RFC2034] if the determined priority level is below
 the lowest priority currently acceptable for the receiving SMTP
 server.  Note that this condition might be temporary.  In some
 environments, operational policies might permit periods of operation
 that relay only higher-priority messages and reject lower priority
 ones.  Such handling choices need to be specified for that
 operational environment.

4.2. Relay of Messages to Other Conforming SMTP/LMTP Servers

 The following rules govern the behavior of a conforming MTA (in the
 role of an SMTP/LMTP client) when relaying a message that was
 received via the SMTP protocol to an SMTP/LMTP server that supports
 the MT-PRIORITY extension:
 1.  An MT-PRIORITY parameter with the value determined by the
     procedure from Section 4.1 MUST appear in the MAIL FROM command
     issued when the message is relayed to an MTA/MDA (Mail Delivery
     Agent) that also supports the MT-PRIORITY extension.  (Note that
     due to site policy, this value might be different from the value
     received from the SMTP client.  See Section 4.1 for details.
     Also note that this value might be different than the priority
     level at which the MTA actually handles the request, due to the
     rounding described in Section 5.)

Melnikov & Carlberg Standards Track [Page 6] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 2.  Further processing of the MT-PRIORITY parameter is described in
     Section 5.

4.3. Relay of Messages to Non-Conforming SMTP/LMTP Servers

 The following rules govern the behavior of a conforming MTA (in the
 role of an SMTP/LMTP client) when relaying a message that was
 received via the SMTP protocol to an SMTP/LMTP server that does not
 support the MT-PRIORITY extension:
 1.  The MTA relays the message without including the MT-PRIORITY
     parameter in the MAIL FROM command.

4.4. Mailing Lists and Aliases

 Several types of mechanisms exist to redirect or forward messages to
 alternative or multiple addresses [RFC5598].  Examples for this are
 aliases and mailing lists [RFC5321].
 If a message is subject to such processing, the Mediator node
 (Section 2.1 of [RFC5598]) SHOULD retain the MT-PRIORITY parameter
 value for all expanded and/or translated addresses.

4.5. Gatewaying a Message into a Foreign Environment

 The following rules govern the behavior of a conforming MTA when
 gatewaying a message that was received via the SMTP protocol into a
 foreign (non-SMTP) environment:
 1.  If the destination environment is unable to provide an equivalent
     of the MT-PRIORITY parameter, the conforming MTA SHOULD behave as
     if it is relaying to a non-conformant SMTP server (Section 4.3).
 2.  If the destination environment is capable of providing an
     equivalent of the MT-PRIORITY parameter, the conforming MTA
     SHOULD behave as if it is relaying to a conformant SMTP server
     (Section 4.2), converting the MT-PRIORITY value to the equivalent
     in the destination environment.

4.6. Interaction with the DSN SMTP Extension

 An MTA that needs to generate a delivery report (whether for
 successful delivery or delayed/failed delivery) for a message it is
 processing SHOULD use the priority value of the message as the
 priority of the generated delivery report.  In particular, this
 requirement applies to MTAs that also implement [RFC3461].

Melnikov & Carlberg Standards Track [Page 7] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 For delivery reports (DSNs) received by an MTA for relay, processing
 rules specified in Section 4.1 apply -- there is no special
 processing for relayed DSNs.  It might seem tempting to try to detect
 DSNs and process them at an elevated priority under the assumption
 that failure notices need to get through quickly, even or perhaps
 especially if the DSN came from an untrusted source.  But such a
 policy can create an exposure to fake DSN attacks by giving untrusted
 systems a way to inject high-priority messages.  Implementation of
 such a policy also assumes that DSNs can be detected reliably, which
 may not be the case since some systems use nonstandard DSN formats.

5. The Priority Service Extension

 The priorities of messages affect the order in which messages are
 transferred from the client to the server.  This is largely
 independent from the order in which they were originally received by
 the server.
 A message priority is a decimal integer in the range from -9 to 9
 (inclusive).  SMTP servers compliant with this specification are not
 required to support all 19 distinct priority levels (i.e., to treat
 each priority value as a separate priority), but they MUST implement
 all distinct priority levels specified in the Priority Assignment
 Policy (see Section 9.2) implemented by the server.  That is, an
 implementation that only supports N priority levels (where N < 19)
 will internally round up a syntactically valid priority value that
 isn't supported to the next higher supported number (or to the
 highest supported priority, if the value is higher than any supported
 priority).  For example, an implementation can treat priority values
 below and including -4 as priority -4, priority -3 as priority -2,
 and all priorities starting from 5 can be treated as priority 6.
 (See Section 9.2 for implementation/deployment considerations related
 to Priority Assignment Policy.)
 Irrespective of the number of distinct priority levels supported by
 the SMTP server, when relaying the message to the next hop or
 delivering it over LMTP, the SMTP server MUST communicate the
 priority value as determined in Section 4.1.
 Note: 19 possible priority levels are defined by this specification
 for extensibility.  For example, a particular implementation or
 deployment environment might need to provide finer-grained control
 over message transfer priorities.  See Appendix E for more details on
 why the range from -9 to 9 was selected.
 As per the Priority Assignment Policy, some SMTP servers MAY impose
 additional maximum message size constraints for different message
 transfer priorities; for example, messages with priority 6 might not

Melnikov & Carlberg Standards Track [Page 8] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 be larger than 4 Kb.  If an SMTP server chooses to reject a message
 because it is too big for the determined priority, it SHOULD use 552
 reply codes together with the X.7.16 Enhanced Status Code [RFC2034].
 Implementation Note: If the SMTP server also supports the SMTP SIZE
 extension [RFC1870], then an SMTP client can use both SIZE= and
 MT-PRIORITY= parameters on the MAIL FROM command.  This allows the
 server to perform early rejection of a message in case the message
 size is too big for the specified priority, thus avoiding wasting
 bandwidth by transferring the message first and then rejecting it due
 to its size.
 The Priority Service Extension can be combined with the DELIVERBY
 [RFC2852] SMTP service extension; however, there is no requirement
 that both extensions always be implemented together.

5.1. Expedited Transfer

 The main service provided by the Priority Message Handling SMTP
 Service Extension is expedited transfer of emails with a higher
 priority.  Therefore, an SMTP client that has more than one email to
 send at a given time sends those with a higher priority before those
 with a lower one.  Additionally, the retry interval and/or default
 timeout before a non-delivery report is generated MAY be lower (more
 aggressive) for messages of higher priority.  Lower retry intervals/
 default timeouts are controlled by the local MTA policy.
 Note that as this SMTP extension requires some sort of trust
 relationship between a sender and a receiver and thus some form of
 authentication (whether using SMTP AUTH, TLS, IP address whitelist,
 etc.), so senders using this SMTP extension will not be subject to
 greylisting [RFC6647], unless they are unauthorized to use this SMTP
 extension due to an explicit policy decision or a misconfiguration
 error.  However, note that in case of connection-level or SMTP EHLO/
 HELO greylisting, SMTP AUTH or TLS authentication options are not
 available to the server.
 In order to make implementations of this extension easier, this SMTP
 extension only allows a single priority for all recipients of the
 same message.
 Within a priority level, the MTA uses its normal algorithm (the
 algorithm used in absence of this SMTP extension) for determining
 message processing order.

Melnikov & Carlberg Standards Track [Page 9] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 Several possible ways of implementing expedited transfer are
 described in more details in Appendix D.  Note that these sections
 don't describe all details and pitfalls for each implementation
 strategy.

5.2. Timely Delivery

 An important constraint (usually associated with higher-priority
 levels) in some environments is that messages with high-priority
 values have some delivery time constraints.  In some cases, higher
 priorities mean a shorter maximum time allowed for delivery.
 Unextended SMTP does not offer a service for timely delivery, i.e.,
 "deliver this message within X seconds from submission" service.  The
 "Deliver By SMTP Service Extension" (DELIVERBY Extension) defined in
 [RFC2852] is an example of an SMTP extension providing a service that
 can be used to implement timely delivery.  Note that SMTP DELIVERBY
 and SMTP MT-PRIORITY extensions are complimentary and can be used
 together (assuming the SMTP server they are talking to advertises
 support for both).  However, note that use of the DELIVERBY extension
 alone does not guarantee any priority processing.  If the client is
 using both SMTP DELIVERBY and SMTP MT-PRIORITY at the same time, the
 client can consider using smaller DELIVERBY timeouts for higher-
 priority messages.

6. Use of MT-PRIORITY with LMTP

 An LMTP server can advertise support for the MT-PRIORITY extension if
 it supports any combination of the following features:
 1.  The LMTP server is architected in such a way that it can deliver
     higher-priority messages quicker than lower-priority messages.
 2.  The LMTP server logs that the MT-PRIORITY extension was used by
     the previous SMTP hop.
 3.  The LMTP server is exposing information about the MT-PRIORITY
     extension to a delivery-time filtering engine such as Sieve
     [RFC5228].

Melnikov & Carlberg Standards Track [Page 10] RFC 6710 Message Transfer Priority SMTP Extension August 2012

7. Syntax

 priority-value = (["-"] NZDIGIT) / "0"
                  ; Allowed values are from -9 to 9 inclusive
 NZDIGIT = %x31-39
           ; "1"-"9"
 CFWS = <defined in RFC 5322>
 ; New "clause" that can be used in the Received header field
 Pri  = CFWS "PRIORITY" FWS priority-value
           ; Complies with the <Additional-Registered-Clauses>
           ; non-terminal syntax from RFC 5321.
 mt-priority-ehlo = "MT-PRIORITY" [SP priority-profile]
           ; Complies with the <ehlo-line> ABNF production
           ; from RFC 5321.
 priority-profile = 1*20(ALPHA / DIGIT / "-" / "_" / ".")
           ; name of the Priority Assignment Profile advertized in
           ; the MT-PRIORITY EHLO response.
 ALPHA = <Defined in RFC 5234>
 DIGIT = <Defined in RFC 5234>

Melnikov & Carlberg Standards Track [Page 11] RFC 6710 Message Transfer Priority SMTP Extension August 2012

8. Example

 The original submission (from MUA (Mail User Agent) to MSA) might
 appear as shown below.  Note that the example is also making use of
 the STARTTLS [RFC3207], DELIVERBY [RFC2852], and DSN [RFC3461] SMTP
 extensions, even though there is no requirement that these other
 extensions be supported when the MT-PRIORITY SMTP extension is
 implemented.
      S: 220 example.com SMTP server here
      C: EHLO mua.example.com
      S: 250-example.com
      S: 250-STARTTLS
      S: 250-AUTH SCRAM-SHA-1 DIGEST-MD5
      S: 250-DSN
      S: 250-DELIVERBY
      S: 250-ENHANCEDSTATUSCODES
      S: 250 MT-PRIORITY MIXER
      C: AUTH SCRAM-SHA-1
      [...authentication exchange...]
      S: 235 2.7.0 Authentication successful
      C: MAIL FROM:<eljefe@example.com> BY=125;R ENVID=QQ314159
          MT-PRIORITY=3
      S: 250 2.1.0 <eljefe@example.com> sender ok
      C: RCPT TO:<topbanana@example.net>
      S: 250 2.1.5 <topbanana@example.net> recipient ok
      C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
          ORCPT=rfc822;Dana@Ivory.example.net
      S: 250 2.1.5 <Dana@Ivory.example.net> recipient ok
      C: DATA
      S: 354 okay, send message
      C:  (message goes here)
      C: .
      S: 250 2.1.0 message accepted
      C: QUIT
      S: 221 2.0.0 goodbye
 In the above example, the MUA has specified the priority 3 and the
 server has accepted it.  The server is advertising the MIXER Priority
 Assignment Policy (the default).  Another variant of the initial
 submission might look like:

Melnikov & Carlberg Standards Track [Page 12] RFC 6710 Message Transfer Priority SMTP Extension August 2012

      S: 220 example.com SMTP server here
      C: EHLO mua.example.com
      S: 250-example.com
      S: 250-STARTTLS
      S: 250-AUTH SCRAM-SHA-1 DIGEST-MD5
      S: 250-DSN
      S: 250-DELIVERBY
      S: 250-ENHANCEDSTATUSCODES
      S: 250 MT-PRIORITY
      C: AUTH SCRAM-SHA-1
      [...authentication exchange...]
      S: 235 2.7.0 Authentication successful
      C: MAIL FROM:<eljefe@example.com> BY=125;R ENVID=QQ314159
      S: 250 2.1.0 <eljefe@example.com> sender ok
      C: RCPT TO:<topbanana@example.net>
      S: 250 2.1.5 <topbanana@example.net> recipient ok
      C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
          ORCPT=rfc822;Dana@Ivory.example.net
      S: 250 2.1.5 <Dana@Ivory.example.net> recipient ok
      C: DATA
      S: 354 okay, send message
      C:  (message goes here)
      C: .
      S: 250 X.3.6 3 is the new priority assigned to the message
      C: QUIT
      S: 221 2.0.0 goodbye
 In the above example, the MUA has not specified any priority, but the
 MSA has assigned priority 3 to the message.  Also note that the
 server is unwilling to adverte the Priority Assignment Policy it
 supports in the EHLO response.
 The MSA relays the message to the next MTA.

Melnikov & Carlberg Standards Track [Page 13] RFC 6710 Message Transfer Priority SMTP Extension August 2012

      S: 220 example.net SMTP server here
      C: EHLO example.com
      S: 250-example.net
      S: 250-DSN
      S: 250-DELIVERBY
      S: 250 MT-PRIORITY STANAG4406
      C: MAIL FROM:<eljefe@example.com> BY=120;R ENVID=QQ314159
          MT-PRIORITY=3
      S: 250 <eljefe@example.com> sender ok
      C: RCPT TO:<topbanana@example.net>
      S: 250 <topbanana@example.net> recipient ok
      C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
          ORCPT=rfc822;Dana@Ivory.example.net
      S: 250 <Dana@Ivory.example.net> recipient ok
      C: DATA
      S: 354 okay, send message
      C:  (message goes here)
      C: .
      S: 250 message accepted
      C: QUIT
      S: 221 goodbye
 The receiving SMTP server advertises support for the "STANAG4406"
 Priority Assignment Policy, which supports 6 priority levels as
 described in Appendix A.  This means that the server will use the
 priority value 4 internally (the next supported priority higher or
 equal to 3) and will communicate the priority value 3 when relaying
 it to the next hop (if necessary).

9. Deployment Considerations

9.1. Multiple MX Records

 If multiple DNS MX records are used to specify multiple servers for a
 domain in Section 5 of [RFC5321], it is strongly advised that all of
 them support the MT-PRIORITY extension and handle priorities in
 exactly the same way.  If one or more servers behave differently in
 this respect, then it is strongly suggested that none of the servers
 support the MT-PRIORITY extension.  Otherwise, unexpected differences
 in message delivery speed or even rejections can happen during
 temporary or permanent failures, which users might perceive as
 serious reliability issues.

Melnikov & Carlberg Standards Track [Page 14] RFC 6710 Message Transfer Priority SMTP Extension August 2012

9.2. Priority Assignment Policies

 This document allows up to 19 distinct priority values.  In a
 particular operating environment, independent originators need to
 assign priority values according to, roughly, the same criteria, so
 that the same "high priority message" doesn't get associated with the
 value 3 for one sender and with the value 5 for another, as such
 messages might unintentionally receive different preferential
 treatment.
 In order to achieve consistent behavior in an operating environment,
 the Priority Assignment Policy (together with possible associated
 restrictions on maximum message sizes for each priority (if any),
 default timeouts, etc.) should be documented for the environment.
 Each SMTP/LMTP server supports a Priority Assignment Policy, whether
 explicit (advertised in the MT-PRIORITY EHLO response) or implicit
 (not advertised).  The default Priority Assignment Policy (assumed by
 the client when no Priority Assignment Policy name is advertised in
 the MT-PRIORITY EHLO response) is specified in Appendix B.  Two other
 policies are specified in Appendix A and Appendix C.  Additional
 policies SHOULD be registered with IANA as specified in Section 10.1.
 Moreover, all MSAs/MTAs/MDAs within any given Administrative
 Management Domain has to be configured to use the same Priority
 Assignment Policy.  Otherwise, a differently configured MSA/MTA/MDA
 can expose the whole domain to possible attacks, like injection of a
 high-priority fake DSN.
 When this SMTP extension is deployed across multiple cooperating
 Administrative Domains, such Administrative Domains need to use the
 same or at least compatible policies.  Again, differences in policies
 (for example, differences in how users are authenticated or
 differences in how priorities are handled) can expose an
 Administrative Domain to weaknesses in a partner domain.

10. IANA Considerations

 IANA has added the MT-PRIORITY SMTP extension to the "SMTP Service
 Extensions" registry
 (http://www.iana.org/assignments/mail-parameters).  This extension is
 suitable for the Submit port.
 IANA has added the following new Received header field clause to the
 "Additional-registered-clauses" sub-registry
 (http://www.iana.org/assignments/mail-parameters) to help with
 tracing email messages delivered using the MT-PRIORITY SMTP
 extension:

Melnikov & Carlberg Standards Track [Page 15] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 Clause name: PRIORITY
 Description: Records the value of the MT-PRIORITY parameter specified
              in the MAIL FROM command
 Syntax of the value: See Section 7 of RFC 6710
 Reference: RFC 6710
 IANA has added the following Enumerated Status Codes to the "Simple
 Mail Transfer Protocol (SMTP) Enhanced Status Codes" registry
 (http://www.iana.org/assignments/smtp-enhanced-status-codes)
 established by [RFC5248]:
 1)  Code:  X.7.15
     Sample Text:  Priority Level is too low
     Associated basic status code:  450, 550 (other 4XX or 5XX codes
        are allowed)
     Description:  The specified priority level is below the lowest
        priority acceptable for the receiving SMTP server.  This
        condition might be temporary, for example the server is
        operating in a mode where only higher-priority messages are
        accepted for transfer and delivery, while lower-priority
        messages are rejected.
     Reference:  RFC 6710
     Submitter:  A. Melnikov
     Change controller:  IESG
 2)  Code:  X.7.16
     Sample Text:  Message is too big for the specified priority
     Associated basic status code:  552 (other 4XX or 5XX codes are
     allowed)
     Description:  The message is too big for the specified priority.
     This condition might be temporary, for example the server is
     operating in a mode where only higher-priority messages below a
     certain size are accepted for transfer and delivery.
     Reference:  RFC 6710
     Submitter:  A. Melnikov
     Change controller:  IESG

Melnikov & Carlberg Standards Track [Page 16] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 3)  Code:  X.3.6
     Sample Text:  Requested priority was changed
     Associated basic status code:  250 or 251
     Description:  The message was accepted for relay/delivery, but
        the requested priority (possibly the implied default) was not
        honored.  The human readable text after the status code
        contains the new priority, followed by SP (space) and
        explanatory human readable text.
     Reference:  RFC 6710
     Submitter:  A. Melnikov
     Change controller:  IESG
 IANA has created a new IANA registry called "SMTP PRIORITY Extension
 Priority Assignment Policy".  Future registrations in this registry
 are governed by the "Specification Required" [RFC5226] IANA
 registration policy.  Requirements on registrations (to be verified
 by the Designated Expert) are specified in Section 10.1.  Changes to
 registrations undergo the same process as initial registrations.  In
 cases of significant changes to registrations (other than editorial
 clarifications), the Designated Expert MAY require registration of a
 Priority Assignment Policy with a new name instead of updating the
 existing one.

10.1. Requirements on Priority Assignment Policy Registrations

 Priority Assignment Policy registrations with IANA are accompanied by
 a policy specification document that MUST specify the following
 information:
 1.  The Priority Assignment Policy name, which is a case-insensitive
     string of 1 to 20 US-ASCII characters to be advertised as the
     MT-PRIORITY EHLO parameter.  Allowed characters are: ALPHA,
     DIGIT, "-", "_", and "."
 2.  Number of distinct priority levels supported by all servers
     implementing the policy and their respective values.
 3.  For each supported priority level: default retry timeouts (how
     often to retry sending a message if there is a temporary error to
     transfer/deliver it).  The policy specification can also
     explicitly define such information as implementation and/or
     deployment specific.

Melnikov & Carlberg Standards Track [Page 17] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 4.  For each supported priority level: default expiration timeouts
     (how long to attempt transfer/delivery before the message expires
     and causes a non-delivery report to be generated).  The policy
     specification can also explicitly define such information as
     implementation and/or deployment specific.  Note that a client
     can override such default when it uses additional SMTP extensions
     (such as the one mentioned in Section 5.2).
 5.  Maximum message size associated with each priority level.  The
     policy specification can also explicitly define such information
     as implementation and/or deployment specific.
 6.  Any requirements/restrictions on the kind of SMTP client
     authentication required in order for an SMTP server implementing
     this policy to accept priority values specified by an SMTP
     client.  For example, this can limit which Simple Authentication
     and Security Layer (SASL) [RFC4422] authentication mechanisms are
     to be used, require TLS, etc.
 7.  Any other information that might affect processing of messages
     with different priorities.
 8.  Note that the policy specification document is not allowed to
     redefine the allowed range of priorities specified in Section 5
     and other aspects of handling of different priorities, unless
     explicitly specified by this document.

10.2. Initial Priority Assignment Policy Registrations

 IANA has registered the following initial values in the "SMTP
 PRIORITY Extension Priority Assignment Policy" registry:
           Initial Priority Assignment Policy Registrations
       +-------------+------------------------+----------------+
       | Policy Name | Reference              | Comment        |
       +-------------+------------------------+----------------+
       | MIXER       | Appendix B of RFC 6710 | Default policy |
       | STANAG4406  | Appendix A of RFC 6710 |                |
       | NSEP        | Appendix C of RFC 6710 |                |
       +-------------+------------------------+----------------+

11. Security Considerations

 Message Submission Agents ought to only accept message transfer
 priorities from users (or only certain groups of such users) who are
 authenticated and authorized in some way that's acceptable to the
 MSA.  As part of this policy, they can also restrict maximum priority

Melnikov & Carlberg Standards Track [Page 18] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 values that different groups of users can request, and can override
 the priority values specified by MUAs.
 Similarly, MTAs ought to only accept message transfer priorities from
 senders (or only certain groups of such senders) who are
 authenticated and authorized in some way that's acceptable to the
 MTA.  As part of this policy, they can also restrict maximum priority
 values that different groups of senders can request, and can override
 the priority values specified by them.
 In the absence of the policy enforcement mentioned above, an SMTP
 server (whether an MSA or an MTA) implementing this SMTP extension
 might be susceptible to a denial-of-service attack.  For example,
 malicious clients (MUAs/MSAs/MTAs) can try to abuse this feature by
 always requesting priority 9.

12. References

12.1. Normative References

 [RFC2033]  Myers, J., "Local Mail Transfer Protocol", RFC 2033,
            October 1996.
 [RFC2034]  Freed, N., "SMTP Service Extension for Returning Enhanced
            Error Codes", RFC 2034, October 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
            Extension for Delivery Status Notifications (DSNs)",
            RFC 3461, January 2003.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
            Mail System Status Codes", BCP 138, RFC 5248, June 2008.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            October 2008.

Melnikov & Carlberg Standards Track [Page 19] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
            October 2008.
 [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            STD 72, RFC 6409, November 2011.

12.2. Informative References

 [ACP123]   CCEB, "Common Messaging strategy and procedures", ACP 123,
            May 2009.
 [PRIORITY-TUNNELING]
            Melnikov, A. and K. Carlberg, "Tunneling of SMTP Message
            Transfer Priorities", Work in Progress, July 2012.
 [RFC1845]  Crocker, D., Freed, N., and A. Cargille, "SMTP Service
            Extension for Checkpoint/Restart", RFC 1845,
            September 1995.
 [RFC1870]  Klensin, J., Freed, N., and K. Moore, "SMTP Service
            Extension for Message Size Declaration", STD 10, RFC 1870,
            November 1995.
 [RFC2156]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
            Mapping between X.400 and RFC 822/MIME", RFC 2156,
            January 1998.
 [RFC2205]  Braden, B., Ed., Zhang, L., Berson, S., Herzog, S., and S.
            Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
            Functional Specification", RFC 2205, September 1997.
 [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS
            Field) in the IPv4 and IPv6 Headers", RFC 2474,
            December 1998.
 [RFC2852]  Newman, D., "Deliver By SMTP Service Extension", RFC 2852,
            June 2000.
 [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, February 2002.
 [RFC4125]  Le Faucheur, F. and W. Lai, "Maximum Allocation Bandwidth
            Constraints Model for Diffserv-aware MPLS Traffic
            Engineering", RFC 4125, June 2005.

Melnikov & Carlberg Standards Track [Page 20] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 [RFC4127]  Le Faucheur, F., Ed., "Russian Dolls Bandwidth Constraints
            Model for Diffserv-aware MPLS Traffic Engineering",
            RFC 4127, June 2005.
 [RFC4190]  Carlberg, K., Brown, I., and C. Beard, "Framework for
            Supporting Emergency Telecommunications Service (ETS) in
            IP Telephony", RFC 4190, November 2005.
 [RFC4412]  Schulzrinne, H. and J. Polk, "Communications Resource
            Priority for the Session Initiation Protocol (SIP)",
            RFC 4412, February 2006.
 [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
            Authentication and Security Layer (SASL)", RFC 4422,
            June 2006.
 [RFC5228]  Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
            Filtering Language", RFC 5228, January 2008.
 [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
            July 2009.
 [RFC6401]  Le Faucheur, F., Polk, J., and K. Carlberg, "RSVP
            Extensions for Admission Priority", RFC 6401,
            October 2011.
 [RFC6647]  Kucherawy, M. and D. Crocker, "Email Greylisting: An
            Applicability Statement for SMTP", RFC 6647, June 2012.
 [SMTP-PRI-OLD]
            Schmeing, M., Brendecke, J., and K. Carlberg, "SMTP
            Service Extension for Priority Message Handling", Work
            in Progress, August 2006.
 [STANAG-4406]
            NATO, "STANAG 4406 Edition 2: Military Message Handling
            System", STANAG 4406, March 2005.

Melnikov & Carlberg Standards Track [Page 21] RFC 6710 Message Transfer Priority SMTP Extension August 2012

Appendix A. Priority Assignment Policy for Military Messaging

 Military Messaging as specified in ACP 123 [ACP123] (also specified
 in STANAG 4406 [STANAG-4406]) defines 6 priority ("precedence")
 values.  While ACP 123/STANAG 4406 allow for 32 different priority
 levels (16 levels are reserved for NATO and an additional 16 are
 reserved for national use), only 6 are in use in practice.  This
 section specifies the Priority Assignment Policy for Military
 Messaging and how the MT-PRIORITY parameter can be mapped when
 gatewaying between SMTP and ACP 123/STANAG 4406 environments.
 Where SMTP is used to support military messaging, the following
 mappings SHOULD be used.
          Recommended Mapping of MT-PRIORITY Values for MMHS
             +-------------------+----------------------+
             | MT-PRIORITY value | MMHS Precedence name |
             +-------------------+----------------------+
             |         -4        | Deferred             |
             |         -2        | Routine              |
             |         0         | Priority             |
             |         2         | Immediate            |
             |         4         | Flash                |
             |         6         | Override             |
             +-------------------+----------------------+
                                Table 1
 The Priority Assignment Policy registration for Military Messaging is
 as follows:
 1.  The Priority Assignment Policy name is "STANAG4406".
 2.  Number of distinct priority levels: 6, as specified in the table
     above.
 3.  Default retry timeouts for each priority level are implementation
     and/or deployment specific.
 4.  Default expiration timeouts for each priority level are
     implementation and/or deployment specific.
 5.  Maximum message size associated with each priority level is
     implementation and/or deployment specific.
 6.  No restrictions on what kind of SMTP client authentication is
     required.

Melnikov & Carlberg Standards Track [Page 22] RFC 6710 Message Transfer Priority SMTP Extension August 2012

Appendix B. Priority Assignment Policy for MIXER

 MIXER [RFC2156] defines the Priority header field with 3 values.
 This section specifies the Priority Assignment Policy for MIXER and
 how the MT-PRIORITY parameter can be mapped when used with MIXER.
 Where SMTP is used to support MIXER messaging, the following mappings
 SHOULD be used.
          Recommended Mapping of MT-PRIORITY Values for MIXER
             +-------------------+----------------------+
             | MT-PRIORITY value | MIXER Priority value |
             +-------------------+----------------------+
             | -4                | non-urgent           |
             | 0                 | normal               |
             | 4                 | urgent               |
             +-------------------+----------------------+
                                Table 2
 The Priority Assignment Policy registration for MIXER is as follows:
 1.  The Priority Assignment Policy name is "MIXER".
 2.  Number of distinct priority levels: 3, as specified in the table
     above.
 3.  Default retry timeouts for each priority level are implementation
     and/or deployment specific.
 4.  Default expiration timeouts for each priority level are
     implementation and/or deployment specific.
 5.  Maximum message size associated with each priority level is
     implementation and/or deployment specific.
 6.  No restrictions on what kind of SMTP client authentication is
     required.

Appendix C. Priority Assignment Policy for National Security /

           Emergency Preparedness (NS/EP)
 There are several forms of communication systems used during an
 emergency or disaster.  The most well known form involves the many-
 to-one model of the general public contacting a public safety access
 point via 911/999/112 calls through the public telephone network.

Melnikov & Carlberg Standards Track [Page 23] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 Typically, these calls do not require authorization, nor do they
 invoke any prioritization.
 Another form of emergency communications involves a set of authorized
 users or nodes that use prioritized services to help establish and
 continue communication given limited available resources.  [RFC4190]
 includes descriptions of several systems that have been developed to
 support National Security / Emergency Preparedness (NS/EP).  These
 deployed systems require a form of authentication and have focused on
 prioritization of telephony-based services.  They have also been
 designed as a binary form (on/off) of signaled priority
 communications.
 [RFC4412] includes examples of a more expansive view of NS/EP
 communications in which priority migrates from a single on/off bit
 value to one that comprises 5 priority values.  This is shown in the
 cases of the Emergency Telecommunications Service (ETS) and Wireless
 Priority Service (WPS) Namespaces.  Given a lack of pre-existing
 NS/EP values assigned for email, we follow the paradigm of the ETS
 and WPS Namespaces and recommend the 5 ascending values shown in the
 table below.
               +-------------------+------------------+
               | MT-PRIORITY value | Relational Order |
               +-------------------+------------------+
               |         -2        | Lowest Priority  |
               |         0         | ----------       |
               |         2         | ----------       |
               |         4         | ----------       |
               |         6         | Highest Priority |
               +-------------------+------------------+
 The Priority Assignment Policy registration for NS/EP is as follows:
 1.  The Priority Assignment Policy name is "NSEP".
 2.  Number of distinct priority levels: 5, as specified in the table
     above.
 3.  Default retry timeouts for each priority level are implementation
     and/or deployment specific.
 4.  Default expiration timeouts for each priority level are
     implementation and/or deployment specific.
 5.  Maximum message size associated with each priority level is
     implementation and/or deployment specific.

Melnikov & Carlberg Standards Track [Page 24] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 6.  No restrictions on what kind of SMTP client authentication is
     required.

Appendix D. Possible Implementation Strategies

 This appendix suggests some strategies to implement the SMTP
 extension defined in this document.  The list is not exhaustive.
 This appendix and its subsections are Informative.

D.1. Probability

 As the name suggests, probability involves increasing the chances of
 obtaining resources without adversely affecting previously
 established connections.  One example would involve requesting
 resources set aside for specific priority levels.  If these
 additional resources are exhausted, then the desired connection is
 denied.  Queues, new timers, or combinations thereof can be used to
 facilitate the higher-priority requests, but the key is that
 mechanisms focus on increasing the probability of message transfer.

D.2. Preemption of Sessions or Transactions

 Preemption is a type of action that focuses only on a comparison of
 priorities to determine if previously established transactions need
 to be displaced in favor of higher-priority requests.  If no
 additional connection is possible, the client aborts a running
 session for emails with lower priority no later than directly after
 the current transaction.  The client can even interrupt an active
 transaction, and ought to do so if other constraints, such as
 delivery time (as specified in the DELIVERBY SMTP extension
 [RFC2852]), would be violated for the email with higher priority.
 When interrupting an active transaction, the client ought to take the
 total message size and the size of the transferred portion of the
 message being interrupted into consideration.  This preliminary
 termination of sessions or transactions is called preemption.
 If preemption of running transactions occurs, the client needs to
 choose a transaction with the lowest priority currently processed.
 If the client has an option (i.e., it is supported by the next-hop
 MTA) to interrupt transactions in a way that allows them to be
 restarted at the interruption point later, it ought to deploy it.  An
 example for a mechanism providing such a service is the "SMTP Service
 Extension for Checkpoint/Restart" defined in [RFC1845].

Melnikov & Carlberg Standards Track [Page 25] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 If a client opts for the preemption of sessions instead of
 transactions, it needs to preempt the next session that reaches the
 end of a transaction.

D.3. Resource Allocation Models

 Adding prioritization to a design moves the subject away from a
 strictly best effort (and a first-come-first-served) model to one
 that includes admission control and resource allocation models.  Over
 the years, a variety of work has been done within the IETF to specify
 resource allocations models.  Examples include the Maximum Allocation
 Model [RFC4125], the Russian Dolls Model [RFC4127], and the Priority
 Bypass Model (Appendix A.3 of [RFC6401]).
 While we recognize that these various models have been designed for
 other protocols (i.e., MPLS and RSVP), an understanding of their
 design characteristics may be beneficial in considering future
 implementations of a priority SMTP service.
 In cases where the processing of high-priority messages by an MTA is
 not considered negligible and exceeds engineered expectations, then
 operators managing that MTA may be notified in some form (e.g.,
 pushed alarm, polled status).

Appendix E. Background on Design Choices

 This section provides some background on design choices made during
 development of the MT-PRIORITY SMTP extension.
 The priority applies per message, rather than per recipient, in order
 to keep the protocol simpler and because of the expectation that it
 will be uncommon to need different priorities for different
 recipients on the same message.  In cases where that is necessary, it
 can always be achieved by sending separate messages with the same
 content, segregating the recipients by desired message priority.
 The choice of the priority range -9 to 9 (as opposed to, say, 1 to 6,
 or 0 to 9) was made after taking the following into consideration:
 1.  Clearly, having multiple priority levels is the whole point of
     this extension.  Existing implementations of similar
     functionality in MTAs are already using 3 levels.  One of the use
     cases motivating this extension requires 6 levels, so at least 6
     different values are required.

Melnikov & Carlberg Standards Track [Page 26] RFC 6710 Message Transfer Priority SMTP Extension August 2012

 2.  During discussions of this extension, several different use cases
     were suggested that required differing numbers of priority
     levels.  Defining just the 6 priority levels needed in item 1,
     above, would limit the extensibility for possible future use
     cases.  Therefore, this document is defining a wider range, which
     allows implementations and deployments to add higher or lower
     priority levels and to insert additional priority levels between
     the recommended set of 6.  This avoids the need to further extend
     this extension just to have a few more priority levels.
 3.  It seems natural to use zero for the "normal" or default
     priority, rather than picking some non-zero number and having the
     priorities go up or down from there.  This way, negative numbers
     always represent priorities that are lower than normal, with
     positive numbers as higher priorities.

Appendix F. Acknowledgements

 This document copies lots of text from "SMTP Service Extension for
 Priority Message Handling" [SMTP-PRI-OLD].  Therefore, the authors of
 this document would like to acknowledge contributions made by the
 authors of that document: Michael Schmeing and Jan-Wilhelm Brendecke.
 Many thanks for input provided by Steve Kille, David Wilson, John
 Klensin, Dave Crocker, Graeme Lunt, Alessandro Vesely, Barry Leiba,
 Bill McQuillan, Murray Kucherawy, SM, Glenn Parsons, Pete Resnick,
 Chris Newman, Ned Freed, and Claudio Allocchio.
 Special thanks to Barry Leiba for agreeing to shepherd this document.

Melnikov & Carlberg Standards Track [Page 27] RFC 6710 Message Transfer Priority SMTP Extension August 2012

Authors' Addresses

 Alexey Melnikov
 Isode Ltd
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex  TW12 2BX
 UK
 EMail: Alexey.Melnikov@isode.com
 Ken Carlberg
 G11
 1601 Clarendon Blvd, #203
 Arlington, VA  22209
 USA
 EMail: carlberg@g11.org.uk

Melnikov & Carlberg Standards Track [Page 28]

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