Request for Comments: 3503
Category: Standards Track
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 © The Internet Society (2003). All Rights Reserved.
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
-- 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].
- 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
- 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
- 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
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
59 Clarendon Road
United Kingdom, WD17 1FQ
Phone: +44 1923 81 2877 EMail: firstname.lastname@example.org
11. Full Copyright Statement
Copyright © The Internet Society (2003). All Rights Reserved.
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