Network Working Group                                        A. Melnikov
Request for Comments: 3503                 ACI Worldwide/MessagingDirect
Category: Standards Track                                     March 2003


          Message Disposition Notification (MDN) profile for
                Internet Message Access Protocol (IMAP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The Message Disposition Notification (MDN) facility defined in RFC
   2298 provides a means by which a message can request that message
   processing by the recipient be acknowledged as well as a format to be
   used for such acknowledgements.  However, it doesn't describe how
   multiple Mail User Agents (MUAs) should handle the generation of MDNs
   in an Internet Message Access Protocol (IMAP4) environment.

   This document describes how to handle MDNs in such an environment and
   provides guidelines for implementers of IMAP4 that want to add MDN
   support to their products.

Table of Contents

   1.  Conventions Used in this Document.............................  2
   2.  Introduction and Overview.....................................  2
   3.  Client behavior...............................................  3
       3.1. Client behavior when receiving a message.................  5
       3.2. Client behavior when copying a message...................  5
       3.3. Client behavior when sending a message...................  5
       3.4. Client behavior when saving a temporary message..........  5
   4.  Server behavior...............................................  5
       4.1. Server that supports arbitrary keywords..................  5
       4.2. Server that supports only $MDNSent keyword...............  5
       4.3. Interaction with IMAP ACL extension......................  6
   5.  Examples......................................................  6
   6.  Security Considerations.......................................  7
   7.  Formal Syntax.................................................  7
   8.  Acknowledgments...............................................  7
   9.  Normative References..........................................  8
   10. Author's Address..............................................  8
   11. Full Copyright Statement......................................  9

1.  Conventions Used in this Document

   "C:" and "S:" in examples show lines sent by the client and server
   respectively.

   The keywords "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in
   this document when typed in uppercase are to be interpreted as
   defined in "Key words for use in RFCs to Indicate Requirement Levels"
   [KEYWORDS].

2.  Introduction and Overview

   This memo defines an additional [IMAP4] mailbox keyword that allows
   multiple Mail User Agents (MUAs) to know if a requested receipt
   notification was sent.

   Message Disposition Notification [MDN] does not require any special
   support of IMAP in the case where a user has access to the mailstore
   from only one computer and is using a single MUA.  In this case, the
   MUA behaves as described in [MDN], i.e., the MUA performs automatic
   processing and generates corresponding MDNs, it performs requested
   action and, with the user's permission, sends appropriate MDNs.  The
   MUA will not send MDN twice because the MUA keeps track of sent
   notifications in a local configuration.  However, that does not work
   when IMAP is used to access the same mailstore from different
   locations or is using different MUAs.

   This document defines a new special purpose mailbox keyword $MDNSent
   that must be used by MUAs.  It does not define any new command or
   response for IMAP, but describes a technique that MUAs should use to
   achieve interoperability.

   When a client opens a mailbox for the first time, it verifies that
   the server is capable of storing the $MDNSent keyword by examining
   the PERMANENTFLAGS response code.  In order to support MDN in IMAP, a
   server MUST support either the $MDNSent keyword, or arbitrary message
   keywords.

3.  Client behavior

   The use of IMAP requires few additional steps in mail processing on
   the client side.  The following timeline modifies the timeline found
   in Section 4 of [MDN].

   -- User composes message.

   -- User tells MUA to send message.

   -- MUA passes message to MSA (original recipient information passed
      along).  MUA [optionally] saves message to a folder for sent mail
      with $MDNSent flag set.

   -- MSA sends message to MTA.

   -- Final MTA receives message.

   -- Final MTA delivers message to MUA (possibly generating DSN).

   -- MUA logs into IMAP server, opens mailbox, verifies if mailbox can
      store $MDNSent keyword by examining PERMANENTFLAGS response.

   -- MUA performs automatic processing and generates corresponding MDNs
      ("dispatched", "processed", "deleted", "denied" or "failed"
      disposition type with "automatic-action" and "MDN-sent-
      automatically" disposition modes) for messages that do not have
      $MDNSent keyword, or \Draft flag set. (*)

   -- MUA sets the $MDNSent keyword for every message that required an
      automatic MDN to be sent, whether or not the MDN was sent.

   -- MUA displays a list of messages to user.

   -- User selects a message and requests that some action be performed
      on it.

   -- MUA performs requested action and, with user's permission, sends
      appropriate MDN ("displayed", "dispatched", "processed",
      "deleted", "denied" or "failed" disposition type with "manual-
      action" and "MDN-sent-manually" or "MDN-sent-automatically"
      disposition mode).  If the generated MDN is saved to a mailbox
      with the APPEND command, the client MUST specify the $MDNSent
      keyword in the APPEND.

   -- MUA sets the $MDNSent keyword for all messages for which the user
      confirmed the dispatching of disposition (or was explicitly
      prohibited to do so).

   -- User possibly performs other actions on message, but no further
      MDNs are generated.

   (*) Note: MUA MUST NOT use \Recent flag as an indicator that it
       should send MDN, because according to [IMAP4], "If multiple
       connections have the same mailbox selected simultaneously, it is
       undefined which of these connections will see newly-arrived
       messages with \Recent set and which will see it without \Recent
       set".  Thus, using \Recent as an indicator will cause
       unpredictable client behavior with different IMAP4 servers.
       However, the client MAY use \Seen flag as one of the indicators
       that MDN must not be sent.  The client MUST NOT use any other
       standard flags, like \Draft or \Answered, to indicate that MDN
       was previously sent, because they have different well known
       meaning.  In any case, in the presence of the $MDNSent keyword,
       the client MUST ignore all other flags or keywords for the
       purpose of generating an MDN and MUST NOT send the MDN.

   When the client opens a mailbox for the first time, it must verify
   that the server supports the $MDNSent keyword, or arbitrary message
   keywords by examining PERMANENTFLAGS response code.

   The client MUST NOT try to set the $MDNSent keyword if the server is
   incapable of storing it permanently.

   The client MUST be prepared to receive NO from the server as the
   result of STORE $MDNSent when the server advertises the support of
   storing arbitrary keywords, because the server may limit the number
   of message keywords it can store in a particular mailbox.  A client
   SHOULD NOT send MDN if it fails to store the $MDNSent keyword.

   Once the $MDNSent keyword is set, it MUST NOT be unset by a client.
   The client MAY set the $MDNSent keyword when a user denies sending
   the notification.  This prohibits all other MUAs from sending MDN for
   this message.

3.1.  Client behavior when receiving a message

   The client MUST NOT send MDN if a message has the $MDNSent keyword
   set.  It also MUST NOT send MDN if a message has \Draft flag, because
   some clients use this flag to mark a message as incomplete.

   See the timeline in section 3 for details on client behavior when
   receiving a message.

3.2.  Client behavior when copying a message

   The client SHOULD verify that $MDNSent is preserved on a COPY
   operation.  Furthermore, when a message is copied between servers
   with the APPEND command, the client MUST set the $MDNSent keyword
   correctly.

3.3.  Client behavior when sending a message

   When saving a sent message to any folder, the client MUST set the
   $MDNSent keyword to prevent another client from sending MDN for the
   message.

3.4.  Client behavior when saving a temporary message

   When saving an unfinished message to any folder client MUST set
   $MDNSent keyword to prevent another client from sending MDN for the
   message.

4.  Server behavior

   Server implementors that want to follow this specification must
   insure that their server complies with either section 4.1 or section
   4.2.  If the server also supports the IMAP [ACL] extension, it MUST
   also comply with the section 4.3.

4.1.  Server that supports arbitrary keywords

   No changes are required from the server to make it compatible with
   the extension described in this document if it supports arbitrary
   keywords.

4.2.  Server that supports only $MDNSent keyword

   Servers that support only the $MDNSent keyword MUST preserve it on
   the COPY operation.  It is also expected that a server that supports
   SEARCH <flag> will also support the SEARCH KEYWORD $MDNSent.

4.3.  Interaction with IMAP ACL extension

   Any server that conforms to either 4.1 or 4.2 and also supports the
   IMAP [ACL] extension, SHOULD preserve the $MDNSent keyword on COPY
   even if the client does not have 'w' right.  This will prevent the
   generation of a duplicated MDN for the same message.  Note that the
   server MUST still check if the client has rights to perform the COPY
   operation on a message according to [ACL].

5.  Examples

   1) MUA opens mailbox for the first time.

   a) The server supports storing of arbitrary keywords

   C: a100 select INBOX
   S: * FLAGS (\Flagged \Draft \Deleted \Seen)
   S: * OK [PERMANENTFLAGS (\Flagged \Draft \Deleted \Seen \*)]
   S: * 5 EXISTS
   S: * 3 RECENT
   S: * OK [UIDVALIDITY 894294713]
   S: a100 OK [READ-WRITE] Completed

   b) The server supports storing of the $MDNSent keyword

   C: a100 select INBOX
   S: * FLAGS (\Flagged \Draft \Deleted \Seen $MDNSent)
   S: * OK [PERMANENTFLAGS (\Flagged \Draft \Deleted \Seen $MDNSent)]
   S: * 5 EXISTS
   S: * 3 RECENT
   S: * OK [UIDVALIDITY 894294713]
   S: a100 OK [READ-WRITE] Completed

   2) The MUA successfully sets the $MDNSent keyword

   C: a200 STORE 4 +FLAGS ($MDNSent)
   S: * 4 FETCH (FLAGS (\Flagged \Seen $MDNSent))
   S: * FLAGS ($MDNSent \Flagged \Deleted \Draft \Seen)
   S: * OK [PERMANENTFLAGS ($MDNSent \Flagged \Deleted \Draft \Seen \*)]
   S: a200 OK STORE completed

   3) The server refuses to store the $MDNSent keyword

   C: a200 STORE 4 +FLAGS ($MDNSent)
   S: a200 NO STORE failed : no space left to store $MDNSent keyword

   4) All clients and servers MUST treat the $MDNSent keyword as case
   insensitive in all operations, as stated in [IMAP].

   C: a300 FETCH 1:* FLAGS
   S: * 1 FETCH (FLAGS (\Seen))
   S: * 2 FETCH (FLAGS (\Answered \Seen $MdnSENt))
   S: * 3 FETCH (FLAGS ())
   S: * 4 FETCH (FLAGS (\Flagged \Seen $MdnSENT))
   S: * 5 FETCH (FLAGS ($MDNSent))
   S: * 6 FETCH (FLAGS (\Recent))
   S: a300 OK FETCH completed
   C: a400 SEARCH KEYWORDS $mdnsent
   S: * SEARCH 2 4 5
   S: a400 OK SEARCH completed

6.  Security Considerations

   There are no known security issues with this extension, not found in
   [MDN] and/or [IMAP4].

   Section 4.3 changes ACL checking requirements on an IMAP server that
   implements IMAP [ACL] extension.

7.  Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) notation as specified in [RFC-822], as modified by
   [IMAP4].  Non-terminals referenced, but not defined below, are as
   defined by [IMAP4].

   Except as noted otherwise, all alphabetic characters are case-
   insensitive.  The use of upper or lower case characters to define
   token strings is for editorial clarity only.  Implementations MUST
   accept these strings in a case-insensitive fashion.

   flag_keyword    ::= "$MDNSent" / other_keywords

   other_keywords  ::= atom

8.  Acknowledgments

   This document is the product of discussions that took place on the
   IMAP mailing list.  Special gratitude to Cyrus Daboo and Randall
   Gellens for reviewing the document.

   Thank you to my father who as he has helped to make me what I am.  I
   miss you terribly.

9.  Normative References

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MDN]      Fajman, R., "An Extensible Message Format for Message
              Disposition Notifications", RFC 2298, March 1998.

   [IMAP4]    Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

   [ACL]      Myers, J., "IMAP4 ACL extension", RFC 2086, January 1997.

10.  Author's Address

   Alexey Melnikov
   ACI Worldwide/MessagingDirect
   59 Clarendon Road
   Watford, Hertfordshire
   United Kingdom, WD17 1FQ

   Phone: +44 1923 81 2877
   EMail: mel@messagingdirect.com

11.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.