IMAP LIST extension for special-use mailboxes
draft-ietf-morg-list-specialuse-01

Abstract

Some IMAP message stores include special-use mailboxes, such as those used to hold draft messages or sent messages. Many mail clients allow users to specify where draft or sent messages should be put, but configuring them requires that the user know which mailboxes the server has set aside for these purposes. This extension adds new mailbox flags that a server MAY include in IMAP LIST command responses to identify special-use mailboxes to the client, easing configuration.

Note

A revised version of this draft document will be submitted to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested, and should be sent to morg@ietf.org.

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Table of Contents

1.  Introduction
1.1.  Conventions used in this document
1.2.  Change log
1.2.1.  Changes from -00 to -01

2.  New mailbox flags identifying special-use mailboxes

3.  Extension to IMAP CREATE command to set special-use flags

4.  Examples
4.1.  Example of an IMAP LIST command
4.2.  Example of an extended IMAP LIST command
4.3.  Example of an IMAP CREATE command

5.  Formal Syntax

6.  Security Considerations

7.  IANA Considerations
7.1.  Registration of CREATE-SPECIAL-USE IMAP capability
7.2.  Registration of SPECIAL-USE return option

8.  References
8.1.  Normative References
8.2.  Informative References

§  Authors' Addresses

1.  Introduction

Some IMAP message stores include special-use mailboxes, such as those used to hold draft messages or sent messages. Many mail clients allow users to specify where draft or sent messages should be put, but configuring them requires that the user know which mailboxes the server has set aside for these purposes. This extension adds new mailbox flags that a server MAY include in IMAP LIST command responses to identify special-use mailboxes to the client, easing configuration.

In addition, this extension adds an OPTIONAL parameter on the IMAP CREATE command, allowing a client to assign a special use to a mailbox when it is created. Servers MAY choose to support this part of the extension, but are not required to.

1.1.  Conventions used in this document

In examples, "C:" indicates lines sent by a client that is connected to a server. "S:" indicates lines sent by the server to the client.

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 RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

1.2.  Change log

[RFC EDITOR] (Please remove this section and its subsections.)

1.2.1.  Changes from -00 to -01

Added wording about list-extended; added reference to RFC 5258; added example; added registration of list-extended return option.

Changed CREATE syntax to use RFC 4466; added reference to RFC 4466; fixed example.

Added "\Archive" use flag.

Noted that multiple mailboxes might have the same flag in some implementations.

Changed some use flag names to be more general: \All (was \AllMail), \Junk (was \Spam), \Flagged (was \Starred).

2.  New mailbox flags identifying special-use mailboxes

An IMAP server that supports this extension MAY include any or all of the following flags in responses to the non-extended IMAP LIST command. The new flags are included along with existing flags, such as "\Marked" and "\Noselect". A given mailbox may have none, one, or more than one of these flags. In some cases, a special use is advice to a client about what to put in that mailbox. In other cases, it's advice to a client about what to expect to find there.

For the extended list command [RFC5258] (Leiba, B. and A. Melnikov, “Internet Message Access Protocol version 4 - LIST Command Extensions,” June 2008.), this extension adds a new return option, "SPECIAL-USE". The new special-use flags SHOULD be returned if the client specifies the SPECIAL-USE return option, and MAY be returned even if the client does not specify it.

The new flags defined here are as follows:

\Drafts

This mailbox is used to hold draft messages -- typically, messages that are being composed but have not yet been sent. In some server implementations, this might be a virtual mailbox, containing messages from other mailboxes that are marked with the "\Draft" message flag. Alternatively, this might just be advice that a client put drafts here.

\Flagged

This mailbox presents all messages marked in some way as "important". When this special use is supported, it is likely to represent a virtual mailbox collecting messages (from other mailboxes) that are marked with the "\Flagged" message flag.

\Inbox

This mailbox is the inbox, where new incoming mail is placed by default. This will normally be the same as the mailbox with the name "inbox", as defined in the base IMAP protocol [RFC3501] (Crispin, M., Ed., “Internet Message Access Protocol,” March 2003.).

\Junk

This mailbox is where messages deemed to be junk mail are held. Some server implementations might put messages here automatically. Alternatively, this might just be advice to a client-side spam filter.

\Sent

This mailbox is used to hold copies of messages that have been sent. Some server implementations might put messages here automatically. Alternatively, this might just be advice that a client save sent messages here.

\Trash

This mailbox is used to hold messages that have been deleted, or marked for deletion. In some server implementations, this might be a virtual mailbox, containing messages from other mailboxes that are marked with the "\Deleted" message flag. Alternatively, this might just be advice that a client that chooses not to use the IMAP "\Deleted" model should use this as its trash location. In server implementations that strictly expect the IMAP "\Deleted" model, this special use is likely not to be supported.

\All

This mailbox presents all messages in the user's message store. Implementations MAY omit some messages, such as, perhaps, those in \Trash and \Junk. When this special use is supported, it is almost certain to represent a virtual mailbox.

\Archive

This mailbox is used to archive messages. The meaning of an "archival" mailbox is server-dependent; typically, it will be used to get messages out of the inbox, or otherwise keep them out of the user's way, while still making them accessible.

All of the above flags are OPTIONAL, and any given server or message store may support any combination of the flags, or none at all. In some server or message store implementations it might be possible for multiple mailboxes to have the same special-use flag. There is no capability string associated with this feature.

3.  Extension to IMAP CREATE command to set special-use flags

As an OPTIONAL feature, a server MAY allow clients to designate a mailbox, at creation, as having one or more special uses. This extension defines the "USE" parameter to the IMAP CREATE command for that purpose (using the syntax defined in RFC 4466 section 2.2 [RFC4466] (Melnikov, A. and C. Daboo, “Collected Extensions to IMAP4 ABNF,” April 2006.)). The new OPTIONAL "USE" parameter is followed by a parenthesized list of zero or more special-use flags, as defined above.

In some server implementations, some special uses may imply automatic action by the server. For example, creation of a "\Junk" mailbox might cause the server to start placing messages that have been evaluated as spam into the mailbox.

In some server implementations, some special uses may result in a mailbox with unusual characteristics or side effects. For example, creation of an "\All" mailbox might cause the server to create a virtual mailbox, rather than a standard one, and that mailbox might behave in unexpected ways (COPY into it might fail, for example).

Servers MAY allow the creation of a special-use mailbox even if one so designated already exists, having the effect of moving the special use from the old mailbox to the new one. Alternatively, servers MAY refuse the creation, considering the designation to be a conflict.

If the server can not create a mailbox with the designated special use defined, for whatever reason, it MUST NOT create the mailbox, and MUST respond to the CREATE command with a tagged NO response.

An IMAP server that supports this OPTIONAL feature will advertise the CREATE-SPECIAL-USE capability string. Clients MUST NOT use the "USE" parameter unless the server advertises the capability.

4.  Examples

4.1.  Example of an IMAP LIST command

This example shows an IMAP LIST response from a server that supports this extension. Note that not all of the flags are used. This server also supports the Child Mailbox extension [RFC3348] (Gahrns, M. and R. Cheng, “The Internet Message Action Protocol (IMAP4) Child Mailbox Extension,” July 2002.).

  C: t1 LIST "" "%"
  S: * LIST (\Marked \HasNoChildren) "/" Inbox
  S: * LIST (\HasNoChildren) "/" ToDo
  S: * LIST (\HasChildren) "/" Projects
  S: * LIST (\Sent \HasNoChildren) "/" SentMail
  S: * LIST (\Marked \Drafts \HasNoChildren) "/" MyDrafts
  S: * LIST (\Trash \HasNoChildren) "/" Trash
  S: t1 OK done

4.2.  Example of an extended IMAP LIST command

This example shows an IMAP LIST response from a server that supports this extension. The client uses the extended IMAP LIST command.

  C: t1 LIST "" "%" RETURN (SPECAL-USE)
  S: * LIST (\Marked) "/" Inbox
  S: * LIST () "/" ToDo
  S: * LIST () "/" Projects
  S: * LIST (\Sent) "/" SentMail
  S: * LIST (\Marked \Drafts) "/" MyDrafts
  S: * LIST (\Trash) "/" Trash
  S: t1 OK done

4.3.  Example of an IMAP CREATE command

This example shows an IMAP CREATE command that might be used to create a mailbox designated to hold draft and sent messages. It also attempts to create a mailbox that will contain all the user's messages, but the server does not support that special use for this user's message store.

  C: t1 CAPABILITY
  S: * CAPABILITY IMAP4rev1 CREATE-SPECIAL-USE
  S: t1 OK done
  C: t2 CREATE MySpecial (USE (\Drafts \Sent))
  S: t2 OK MySpecial created
  C: t3 CREATE Everything (USE (\All))
  S: t3 NO \All not supported

5.  Formal Syntax

The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in [RFC5234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).

create-param =/

"USE" SP "(" [use-flag *(SP use-flag)] ")" ; Extends "create-param" from RFC 4466 [RFC4466] (Melnikov, A. and C. Daboo, “Collected Extensions to IMAP4 ABNF,” April 2006.)

mbx-list-oflag =/

use-flag ; Extends "mbx-list-oflag" from IMAP base [RFC3501] (Crispin, M., Ed., “Internet Message Access Protocol,” March 2003.)

return-option =/

"SPECIAL-USE" ; Extends "return-option" from
; LIST-extended [RFC5258] (Leiba, B. and A. Melnikov, “Internet Message Access Protocol version 4 - LIST Command Extensions,” June 2008.)

use-flag =

"\All" / "\Archive" / "\Drafts" / "\Flagged" / "\Inbox" / "\Junk" / "\Sent" / "\Trash" / use-flag-ext

use-flag-ext =

"\" atom ; Reserved for future extensions. Clients
; MUST ignore list flags they do not understand
; Server implementations MUST NOT generate
; extension flags except as defined by
; future standards-track revisions of or
; extensions to this specification.

6.  Security Considerations

LIST response: There are no security issues with conveying special-use information to a client.

CREATE command "USE" parameter: In some server implementations, some special uses may imply automatic action by the server. For example, creation of a "\Junk" mailbox might cause the server to start placing messages that have been evaluated as spam into the mailbox. Server implementors SHOULD consider the consequences of allowing a user (or client program) to designate the target of such automatic action.

7.  IANA Considerations

7.1.  Registration of CREATE-SPECIAL-USE IMAP capability

This document defines a new IMAP capability. IANA is asked to add "CREATE-SPECIAL-USE" to the imap4-capabilities registry.

7.2.  Registration of SPECIAL-USE return option

This document defines a new IMAP4 List Extended return option. IANA is asked to add "SPECIAL-USE" to the imap4-list-extended registry, as follows:

To: iana@iana.org
Subject: Registration of LIST-EXTENDED option SPECIAL-USE
LIST-EXTENDED option name: SPECIAL-USE
LIST-EXTENDED option type: RETURN
LIST-EXTENDED option description: Request special-use mailbox information
Published specification: [[this RFC]]
Security considerations: none
Intended usage: COMMON
Person and email address to contact for further information: Authors' Addresses at the end of [[this RFC]]
Owner/Change controller: iesg@ietf.org

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses