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Versions: 00 01 02                                                      
Internet Engineering Task Force                               J. Kundrat
Internet-Draft                                           August 27, 2013
Intended status: Standards Track
Expires: February 26, 2014

                         IMAP SUBMIT Extension


   This document extends the IMAP protocol with a feature to submit
   e-mail messages for delivery.  It is intended to serve as a better
   alternative to the URLAUTH/BURL approach.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 26, 2014.

Copyright Notice

   Copyright (c) 2013 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Mode of Operation  . . . . . . . . . . . . . . . . . . . . . .  3

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   3.  IMAP Protocol Changes  . . . . . . . . . . . . . . . . . . . .  3
     3.1.  New IMAP Capabilities  . . . . . . . . . . . . . . . . . .  4
       3.1.1.  The SUBMIT Capability  . . . . . . . . . . . . . . . .  4
       3.1.2.  The SUBMIT= Capabilities Family  . . . . . . . . . . .  4  SUBMIT=DSN . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Additional Response Codes  . . . . . . . . . . . . . . . .  4
       3.2.1.  The POLICYDENIED Response Code . . . . . . . . . . . .  4
       3.2.2.  The SUBMISSIONRACE Response Code . . . . . . . . . . .  4
     3.3.  UID SUBMIT command . . . . . . . . . . . . . . . . . . . .  5
       3.3.1.  Delivery options . . . . . . . . . . . . . . . . . . .  6  FROM Delivery Option . . . . . . . . . . . . . . .  6
   DSN-ENVID Submission Option  . . . . . . . . .  6  RECIPIENT Delivery Option  . . . . . . . . . . . .  7
   DSN Submission Option  . . . . . . . . . . . .  7
   DSN-RET Submission Option  . . . . . . . . . .  9
   4.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Appendix A. FIXME Items  . . . . . . . . . . . . . . . . . . . . . 13
   Appendix B. Changelog  . . . . . . . . . . . . . . . . . . . . . . 13
     Appendix B.1.  Changes in -02 since -01  . . . . . . . . . . . . 13
     Appendix B.2.  Changes in -01 since -00  . . . . . . . . . . . . 13
     Appendix B.3.  Changes in -00 since private-01 . . . . . . . . . 13
     Appendix B.4.  Changes in private-01 since private-00  . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   In the traditional IMAP/ESMTP service model, a MUA transfers each
   outgoing message twice -- once for storing it in the user's "sent
   mail" folder, and the second time for actual message submission over
   (E)SMTP.  Under certain circumstances, such as when the message
   contains data which are already available in another message stored
   on the same IMAP server (such as when forwarding an unread attachment
   to another recipient), the MUA has to download the data before the
   message can be composed, resulting in transmitting the data three
   times in total.

   Many proposals exist which aim to reduce this high number of
   transfers to the lowest possible number.  The CATENATE extension
   [RFC4469] enables IMAP clients to have the IMAP servers compose
   messages on their behalf, optionally using data already available on
   the IMAP server.  Using CATENATE, MUAs do not have to download
   individual message parts before including them to the newly created

   The LEMONADE extension family [RFC5550] mandates full support for
   BURL [RFC4468] and URLAUTH [RFC4467] extensions.  When coupled with a
   properly configured pair of ESMTP and IMAP servers, these two
   extensions allow MUAs to have the submission server obtain the
   message payload from the IMAP server.  This approach completely

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   eliminates the need to upload the message data to the ESMTP server,
   achieving the "forward-without-download" goal.

   The BURL/URLAUTH extensions, however, put a significant burden on the
   server operators who suddenly have to establish an explicit trust
   relation between their submission and IMAP servers, and make this
   trust path visible to the users' MUAs.  No MUA-visible means of
   discovering this trust relation are defined.  Furthermore, the whole
   scheme still requires the MUAs to maintain two distinct connections
   speaking different protocols.  Users are prompted for two sets of
   credentials to authenticate to each of these two services.  Real-
   world support issues were reported where users are able to access
   their IMAP service while access to the submission service is blocked
   by a mis-configured firewall.

   The SUBMIT extension of the IMAP protocol effectively moves the
   message submission process to be initiated by a user's request to
   their IMAP server.  When deployed, this scenario provides a perfect
   discoverability to the users' MUAs, saves the overhead of
   establishing and securing a separate TCP connection against the
   submission server, reduces the amount of the configuration data the
   users are required to provide, and simplifies the trust paths which
   are required to exist between the submission and the IMAP servers.
   When combined with the existing CATENATE extension [RFC4469], the
   SUBMIT command works at least as effectively as the Lemonade trio of

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Mode of Operation

   The SUBMIT extension adds the UID SUBMIT IMAP command which instructs
   the IMAP server to arrange for delivery of an already existing IMAP
   message.  How this message is composed is outside of scope of this
   extension, but it is assumed that clients will often use the APPEND
   or APPEND ... CATENATE commands.

   Upon receiving the SUBMIT command, the IMAP server is asked for
   arranging the initial message submission.  Clients MAY pass
   additional data in form of various options of the SUBMIT command (the
   delivery options).  The server checks the passed data and delivery
   options, optionally performs sanity checks on the message contents,
   verifies against a local policy whether the user is authorized for
   message submission, and if none of these checks fail, the server
   passes the message for subsequent delivery.  The delivery method is
   outside of scope of this document, but typical methods would be
   invoking a `sendmail`-compatible binary or passing the message to an
   ESMTP gateway.

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3.  IMAP Protocol Changes

   This extension introduces one new IMAP command, a few related
   response codes and a new family of the IMAP capabilities.

3.1.  New IMAP Capabilities

3.1.1.  The SUBMIT Capability

   Servers implementing this extension announce its presence through the
   SUBMIT capability.  If the server supports this extension but message
   submission is unconditionally disabled by a security policy or
   service configuration, this capability MUST NOT be announced.

   When this capability is present, clients may assume that chances are
   high that submitting messages over IMAP will work.

3.1.2.  The SUBMIT= Capabilities Family

   The SUBMIT commands issued by the IMAP clients MAY contain delivery
   options, and these options might contain other fine grained options.
   Servers supporting voluntary features MUST indicate so by including
   the appropriate strings in the CAPABILITY responses.  All
   capabilities used for these purposes begin with the SUBMIT= prefix.  SUBMIT=DSN

   If the server supports user control of generating the Delivery Status
   Notifications (DSN), it MUST announce the SUBMIT=DSN capability.
   Clients MUST NOT attempt to control DSN options through the DSN
   submission option unless the server announces the SUBMIT=DSN

3.2.  Additional Response Codes

   The following response codes are defined for communicating the reason
   why submission failed in a machine-readable way.

3.2.1.  The POLICYDENIED Response Code

   The POLICYDENIED response code SHOULD be used if the server rejects
   message submission as a result of a policy based decision which MAY
   take the message content, submission options, user's behavior and
   transaction history into account.

   Upon seeing the POLICYDENIED response code, the client MUST inform
   the user that message submission failed, and include the text of the
   response in the error message.  Clients MUST NOT attempt to
   automatically re-queue this message for sending over IMAP.  The
   clients MAY, however, choose to continue the message submission by
   another channel, perhaps over an ESMTP service.

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3.2.2.  The SUBMISSIONRACE Response Code

   The SUBMISSIONRACE response code MUST be sent in the tagged response
   if the client asks for submission of a message that is either not
   marked with the $SubmitPending keyword or marked with the $Submitted

   The SUBMISSIONRACE response code could SHOULD have a preference over
   the POLICYDENIED response code.

3.3.  UID SUBMIT command

   The UID SUBMIT command submits a message for delivery.


   o  UID of message to be sent

   o  optional list of delivery options

   Responses: FETCH response with updated message flags


   OK Message submitted for delivery

   NO Submission failed

   BAD Invalid commands or options

   This command is only valid in the selected state.

   The server MUST check its local policy configuration and verify that
   the authenticated user is allowed to submit messages.  The decision
   MAY be based on the user's credentials, the message contents, past
   history of the user, or any other criteria the server deems relevant.
   The server SHOULD take into account any other local policies before
   it proceeds with further submission.

   Clients MUST NOT submit a message which is either not marked with the
   $SubmitPending keyword from [RFC5550], or which is marked with the
   $Submitted keyword.  Servers MUST reject such a command with a tagged
   NO bearing the SUBMISSIONRACE response code.

   In the course of submission, servers MUST atomically add the
   $Submitted flag to the message, as described in LEMONADE [RFC5550].
   A transient state where the message is temporarily marked with both
   $Submitted and $SubmitPending flags MAY be hidden from any IMAP
   session or it MAY be visible in some or all of them.

   If the command succeeded, the message MUST be marked with the
   $Submitted keyword, the $SubmitPending keyword MUST be cleared and a
   FETCH response containing the message UID and its new flags MUST be

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   If the command fails, the server MUST clear both the $Submitted or
   $SubmitPending keywords.

   If the server supports CONDSTORE [RFC4551] or QRESYNC [RFC5172]
   extensions, any flag changes MUST obey the usual MODSEQ invariants.
   Any changes in the MODSEQ values MUST be communicated to the clients,
   as mandated by the relevant extensions.

   Clients MUST be prepared to handle failing submission at any time.
   Servers MAY reject message submission for any reason.

   The server MUST process all specified delivery options and their
   detailed options.  The server MUST respond with a tagged BAD if the
   client used unrecognized or unannounced option, or if a recognized
   option is used in an invalid way.  If the server cannot honor a
   recognized and announced option, it MUST respond with a tagged NO
   with the POLICYDENIED response code and the message MUST NOT be
   submitted, nor its flags changed.

   Servers MAY alter the message payload of the outgoing message in
   conformance with best current practice about Internet mail.
   Individual recipients MAY receive different versions of the message.
   In particular, servers MUST change message headers so that the
   identity of addresses present in the Bcc headers is not revealed to
   other recipients.  This mode of operation is described in [RFC5321]
   and [RFC5322].  The copy stored on the IMAP server MUST NOT be
   altered by these modifications.

3.3.1.  Delivery options

   The following two delivery options are defined by this extension.
   These options apply to the message as a whole:  FROM Delivery Option

   Syntax: one e-mail address with optional further data

   The FROM delivery option corresponds to the FROM field of the SMTP
   envelope.  If not present, its value MUST be inferred from the
   message payload.

   It is an error if the FROM delivery option is present more than once.
   Servers MUST reject such commands using the BAD tagged response and
   the message MUST NOT be submitted.  Message flags of the source
   message MUST NOT be modified.

   The following per-message submission option is defined by this
   extensions:  DSN-ENVID Submission Option

   Syntax: specification of ESMTP Envelope ID

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   This per-message submission option corresponds to the ENVID=...
   parameter from [RFC3461].  It allows senders to attach a machine-
   readable ID to be received in the delivery status reports concerning
   this message.

   Clients MUST NOT use the DSN-ENVID return option unless the server
   announces the SUBMIT=DSN capability or a SUBMIT=... capability
   defined by future extensions which make use of the ENVID ESMTP
   parameter.  RECIPIENT Delivery Option

   Syntax: one e-mail address followed by optional further data

   The RECIPIENT delivery option corresponds to the RCPT TO field of the
   SMTP envelope.

   The RECIPIENT delivery option MAY be present more than once.  Servers
   MAY impose a limit on the number of recipients of a single message.

   If the RECIPIENT delivery option is present, servers MUST ignore any
   To, Cc and Bcc headers in the message payload when determining the
   list of recipients of this message.  That is, the final list of
   recipients of the message MUST consist exactly of those recipients
   specified in the RECIPIENT delivery options.  The server MUST still
   sanitize the headers of the submitted message to guarantee the
   privacy of the recipients listed in the Bcc message header.

   If the RECIPIENT delivery option is missing, servers MUST infer its
   value from the message payload.  For example, each address present in
   any of To, Cc and Bcc message headers MUST be present in the
   recipient list.

   Servers MAY impose a local policy decision about always sending a
   copy of message to a certain address.  This operation MUST NOT remove
   any addresses from the list of recipients, as obtained either from
   the user-specified list of recipients passed through the RECIPIENTS
   delivery options, or inferred from the mail headers.

   Message submission MUST be atomic -- message is either submitted for
   delivery to all recipients, or it MUST NOT be submitted for delivery
   to anyone.  Actual delivery MAY still fail for certain recipients, as
   per usual ESMTP semantics.

   The following submission options are defined for the RECIPIENT
   delivery option:  DSN Submission Option

   Syntax: delivery status notice specification

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   The DSN submission option controls generating of delivery status
   notifications related to the currently submitted message.  When not
   given, an implementation-defined default value MUST be used.  The
   default value MUST be either (FAILURE) or (DELAY, FAILURE), as

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   mandated by [RFC3461].

   It is an error if the DSN submission option is present multiple times
   for one recipient.

   Clients MUST NOT specify the DSN submission option unless the server
   announces the SUBMIT=DSN capability.  Support for the SUBMIT=DSN
   submission option is OPTIONAL.

   The DSN specification is either "NONE" to deactivate DSNs altogether,
   or a parenthesized list of any of the following DSN options:

   SUCCESS requests generating DSNs upon successful delivery of a

   DELAY activates generating DSNs when delivery is delayed

   FAILURE requests generating DSNs when the delivery fails

   The order of DSN requests is not significant.  A single DSN option
   item MUST NOT be repeated in the DSN specification for one recipient.  DSN-RET Submission Option

   Syntax: DSN return option specification

   This per-recipient submission option corresponds to the RET=...
   parameter from [RFC3461].  Two values are defined, "HDRS" and "FULL",
   meaning to include only the headers or the full message,
   respectively, in the generated delivery status reports.

   Clients MUST NOT use the DSN-RET return option unless the server
   announces the SUBMIT=DSN capability.

4.  Example

   This section contains an example of how message submission over IMAP

   The following example shows how client should submit a message with
   UID 123 in the current mailbox for delivery.  If the message is
   passed through SMTP, its From address in the SMTP envelope MUST be
   set to foo@example.org and it MUST be submitted for delivery to two
   recipients, the a@example.org and b@example.org.  The DSN options are
   set to report about excess delays and failures in message delivery
   for the first recipient.  System's default policy of DSN production
   is retained for the second recipient.

         C: x UID SUBMIT 123 (FROM "foo@example.org"
                 RECIPIENT "a@example.org" DSN (delay failure)
                 RECIPIENT "b@example.org"
         S: * 10 FETCH (UID 123 FLAGS ($Submitted))
         S: x OK Message passed to the sendmail binary

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   In the following example, a message is delivered to addresses
   specified in the message payload.  No delivery options are given, and
   therefore the From and To envelope items are inferred from the actual
   payload.  The DSN options, if supported, are set to an
   implementation-defined default value.

             C: x UID SUBMIT 123
             S: * 10 FETCH (UID 123 FLAGS ($Submitted))
             S: x OK Message passed to the sendmail binary

5.  Acknowledgements


6.  IANA Considerations

   IMAP4 capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  The registry is currently located


   This document defines the following list of IMAP capabilities.  IANA
   will be asked to add them to the registry:

   o  SUBMIT


   FIXME: response codes

7.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234].

   Non-terminals referenced but not defined below are as defined by
   [RFC3501], or [RFC5234].

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   capability          =/ "SUBMIT" / "SUBMIT=DSN"
                         ;; This extension also reserves all further
                         ;; capabilities starting with the "SUBMIT="
                         ;; prefix for future extensions related to the
                         ;; message submission over IMAP

   uid                 =/ "UID" SP sendmail

   sendmail            = "SUBMIT" SP uniqueid [SP delivery-options]

   delivery-options    = "(" delivery-option *( SP delivery-option ) ")"

   delivery-option     = submission-from / submission-recipient

   submission-from     = "FROM" SP one-email-addr [SP mail-from-params]
                         ;; MUST NOT be present more than once
                         ;; in the delivery-options block

   submission-recipient= "RECIPIENT" SP one-email-addr [SP mail-rcpt-params]
                         ;; MAY be present more than once

   mail-from-params    = "(" mail-from-param *( SP mail-from-param ) ")"

   mail-from-param     = sub-option-dsn-envid / sub-generic-option

   mail-rcpt-params    = "(" mail-rcpt-param *( SP mail-rcpt-param ) ")"

   mail-rcpt-param     = sub-option-rcpt-dsn / sub-option-dsn-ret
                         / sub-generic-option

   sub-generic-option  = string SP string
                         ;; FIXME: do we want to retain this in without any semantics?

   sub-option-rcpt-dsn = "DSN" SP ( "NONE" / dsn-spec )
                         ;; MUST NOT be present more than once for each recipient

   dsn-spec            = "(" dsn-spec-item *( SP dsn-spec-item ) ")"
                         ;; an individual dsn-spec-item MUST NOT
                         ;; be present more than once in this list

   dsn-spec-item       = "DELAY" / "FAILURE" / "SUCCESS"

   sub-option-dsn-ret  = "DSN-RET" SP ( "FULL" / "HDRS" )
                         ;; MUST NOT be present more than once for each recipient

   sub-option-dsn-envid= "DSN-ENVID" SP xtext
                         ;; MUST NOT be present more than once
                         ;; <xtext> is defined in [RFC3461], section 4.
                         ;; The allowed syntax is further limited by
                         ;; its section 4.4.

   one-email-addr      = string
                         ;; valid e-mail address as per [RFC5321]

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8.  Security Considerations

   This extension introduces a new way of allowing authenticated users
   to submit Internet mail.  Servers supporting this extension SHOULD
   implement the same security measures as other SUBMISSION [RFC4409]
   servers open to users.

   The redirect command from SIEVE [RFC5228] already requires some types
   of IMAP message stores to be able to generate outgoing mail.
   Security considerations for this extension are similar.

   For the IMAP-based submission to work, the server operators MUST
   configure their MTA systems to accept submission requests from their
   IMAP servers.  This change MAY have security implications, even
   though services supporting the SIEVE filtering are already configured
   to accept e-mails for submission.

   The new trust path MAY replace the trust path required for the BURL/
   URLAUTH operation required by the LEMONADE extension family.

9.  References

   [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.

              4rev1", RFC 3501, March 2003.

   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.

   [RFC4467]  Crispin, M., "Internet Message Access Protocol (IMAP) -
              URLAUTH Extension", RFC 4467, May 2006.

   [RFC4468]  Newman, C., "Message Submission BURL Extension", RFC 4468,
              May 2006.

   [RFC4469]  Resnick, P., "Internet Message Access Protocol (IMAP)
              CATENATE Extension", RFC 4469, April 2006.

   [RFC4551]  Melnikov, A. and S. Hole, "IMAP Extension for Conditional
              STORE Operation or Quick Flag Changes Resynchronization",
              RFC 4551, June 2006.

   [RFC5172]  Varada, S., "Negotiation for IPv6 Datagram Compression
              Using IPv6 Control Protocol", RFC 5172, March 2008.

   [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5550]  Cridland, D., Melnikov, A. and S. Maes, "The Internet
              Email to Support Diverse Service Environments (Lemonade)
              Profile", RFC 5550, August 2009.

Appendix A.  FIXME Items

   IANA and the response codes

   "if the command fails, server MUST clear both $SubmitPending and
   $Submitted" -- what to do when there's something like a disk error?

Appendix B.  Changelog

Appendix B.1.  Changes in -02 since -01

   o  Clarified priorities of SUBMISSIONRACE and POLICYDENIED

   o  Clarify failover procedure upon seeing a POLICYDENIED

   o  Clarified message flag manipulation

   o  Clarified recipient list mangling

   o  More editorial work

Appendix B.2.  Changes in -01 since -00

   o  "Delivery options" is the new name for former "submission
      options"; changed the existing DSN options family to use this new
      syntax (thanks to Arnt for this suggestion)

   o  Clarified the visibility of the transitional state with both
      $Submitted and $SubmitPending

   o  Some editorial work

   o  Fixed an error in the example (multiple FROM addresses...)

Appendix B.3.  Changes in -00 since private-01

   o  Renamed to SUBMIT

   o  DSNs are per-recipient, not per-message

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   o  The introduction was rewritten

   o  Miscellaneous clarifications

   o  Changed DSN NIL to DSN NONE

   o  Clarified the semantics of the RECIPIENT submission option to
      guarantee Bcc privacy

   o  Editorial tweaks, including changes to the required SHOULD/MUST/

   o  DSN's ENVID and RET

Appendix B.4.  Changes in private-01 since private-00

   o  Removed the superfluous SENDER submission option

   o  Mandating Bcc removal in conformance with RFC 5321 / RFC 5322

Author's Address

   Jan Kundrat
   Eledrova 558
   Prague 181 00

   Email: jkt@flaska.net

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