Network Working Group B. Leiba, Ed.
Internet-Draft Huawei Technologies
Intended status: Standards Track November 14, 2017
Expires: May 18, 2018

IMAP $Important Keyword and \Important Special-Use Attribute


RFC 6154 created an IMAP Special-Use LIST extension and defined an initial set of attributes. This document defines a new attribute, "\Important", and establishes a new IANA registry for IMAP folder attributes, registering the attributes defined in RFCs 3348, 3501, and 6154. This document also defines a new IMAP keyword, "$Important", and registers it in the registry defined in RFC 5788.

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Table of Contents

1. Introduction

The Internet Message Access Protocol (IMAP) specification [RFC3501] defines the use of message keywords, and an IMAP Keywords registry is created in [RFC5788]. [RFC6154] defines an extension to the IMAP LIST command for special-use mailboxes. The extension allows servers to provide extra information (attributes) about the purpose of a mailbox and defines an initial set of special-use attributes.

This document does the following:

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.

2. Definition of the '$Important' Message Keyword

The "$Important" keyword is a signal that a message is likely important to the user. The keyword can be set by the user, or automatically by the system based on available signals (such as who the message is from, who else the message is addressed to, evaluation of the subject or content, or other heuristics).

This is distinct from the "\Flagged" system flag in two ways:

  1. "$Important" carries a specific meaning of importance, as opposed to urgency. It is meant to be used for a form of triage, with "\Flagged" remaining as a designation of special attention or particular urgency.
  2. The setting of "$Important" is expected to be based at least partly on heuristics, whereas "\Flagged" is intended to be set by the user.

3. Definition of the 'Important' Mailbox Attribute

The "\Important" mailbox attribute is a signal that the mailbox contains messages that are likely important to the user. In an implementation that also supports the "$Important" keyword, this special use is likely to represent a virtual mailbox collecting messages (from other mailboxes) that are marked with the "$Important" keyword. In other implementations, the system might automatically put messages there based on the same sorts of heuristics that are noted for the "$Important" keyword (see Section 2). The distinction between "\Important" and "\Flagged" for mailboxes is similar to those between "$Important" and "\Flagged" for messages.

3.1. Formal Syntax

    use-attr      =/  "\Important"

The following syntax specification adds to the one in [RFC6154], Section 6, using Augmented Backus-Naur Form (ABNF) as described in [RFC5234].

3.2. Example

    C: t1 list "" "Imp*"
    S: * LIST (\HasNoChildren \Important) "/" "Important Messages"
    S: * LIST (\HasNoChildren) "/" "Imported Wine"
    S: t1 OK Success

In the following example, the mailbox called "Important Messages" is the one designated with the "\Important" attribute.

4. Security Considerations

The security considerations in [RFC6154], Section 7, apply equally to this extension. In particular, "Conveying special-use information to a client exposes a small bit of extra information that could be of value to an attacker." Moreover, identifying "important" messages or a place where important messages are kept could give an attacker a strategic starting point. If the algorithm by which messages are determined to be important is well known, still more information is exposed -- perhaps, for example, there is an implication that the senders of these messages are particularly significant to the mailbox owner, and perhaps that is information that should not be made public.

As noted in RFC 6154, it is wise to protect the IMAP channel from passive eavesdropping, and to defend against unauthorized discernment of the identity of a user's "\Important" mailbox or of a user's "$Important" messages.

5. IANA Considerations

This document contains 3 actions for IANA, specified in the sections below:

  1. Registration of the "$Important" keyword.
  2. Creation of a new "IMAP Mailbox Name Attributes" registry.
  3. Registration of initial entries in the "IMAP Mailbox Name Attributes" registry.

5.1. Registration of the $Important keyword

IANA is asked to register the $Important keyword in the "IMAP Keywords" registry, as follows, using the template in [RFC5788].

IMAP keyword name:
Purpose (description):
The "$Important" keyword is a signal that a message is likely important to the user.
Private or Shared on a server:
Is it an advisory keyword or may it cause an automatic action:
Advisory (but see the reference for details).
When/by whom the keyword is set/cleared:
The keyword can be set by the user, or automatically by the system based on available signals (such as who the message is from, who else the message is addressed to, evaluation of the subject or content, or other heuristics).
Related keywords:
None (but see the reference for the related mailbox name attribute).
Related IMAP capabilities:
Security considerations:
See [[THIS RFC]], Section 4
Published specification:
Person & email address to contact for further information:
IETF Applications Area <>
Intended usage:
Owner/Change controller:

5.2. Creation of the IMAP Mailbox Name Attributes Registry

IANA is asked to create a new registry in the group "Internet Message Access Protocol (IMAP)". The new registry will be called "IMAP Mailbox Name Attributes", and will have two references: "RFC 3501, Section 7.2.2", and "[[THIS RFC]], Section 5".

The registry entries will contain three fields:

  1. Attribute Name
  2. Description
  3. Reference

IANA will keep this list in alphabetical order by Attribute Name, which is registered without the initial backslash ("\").

The registration policy for the new registry will be listed as "IETF Review or Expert Review" [RFC8126], and new registrations will be accepted in one of two ways:

  1. For registrations requested in an IETF consensus document, the registration policy will be IETF Review, and the request will be made in the IANA Considerations section of the document, giving the requested values for each of the three fields.
  2. For other registrations, the policy will be Expert Review policy (see Section 5.2.1), and the request will be made by sending email to IANA asking for a new IMAP Mailbox Name Attribute and giving the requested values for each of the three fields.

5.2.1. Instructions to the Designated Expert

The expert reviewer, who will be designated by the IESG, is expected to provide only a general review of the requested registration, checking that the reference and description are adequate for understanding the intent of the registered attribute. Efforts should also be made to generalize the intent of an attribute so that multiple implementations with the same requirements may reuse existing attributes. Except for this check, this is intended to be very close to a first come first served policy, and the expert should not block serious registration requests with a reasonable reference. The reference may be to any form of documentation, including a web page, but consideration should be given to providing one that is expected to be long-lived and stable.

5.3. Initial Entries for the IMAP Mailbox Name Attributes Registry

| Attribute     | Description                       | Reference |
| Name          |                                   |           |
| All           | All messages                      | [RFC6154] |
| Archive       | Archived messages                 | [RFC6154] |
| Drafts        | Messages that are working drafts  | [RFC6154] |
| Flagged       | Messages with the \Flagged flag   | [RFC6154] |
| HasChildren   | Has accessible child mailboxes    | [RFC3348] |
| HasNoChildren | Has no accessible child mailboxes | [RFC3348] |
| Important     | Messages deemed important to user | THIS RFC  |
| Junk          | Messages identified as Spam/Junk  | [RFC6154] |
| Marked        | Server has marked the mailbox as  | [RFC3501] |
|               | "interesting"                     |           |
| NoInferiors   | No hierarchy under this name      | [RFC3501] |
| Noselect      | The mailbox is not selectable     | [RFC3501] |
| Sent          | Sent mail                         | [RFC6154] |
| Trash         | Messages the user has discarded   | [RFC6154] |
| Unmarked      | No new messages since last select | [RFC3501] |

The registry will initially contain these entries:

6. Changes During Document Development

[CREF1]RFC Editor: Please remove this section prior to publication.

Changes in -02

Changes in -01

7. Contributors

The following author was an original contributor to this document in addition to the editor.

Eric "Iceman"

8. References

8.1. Normative References

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC6154] Leiba, B. and J. Nicolson, "IMAP LIST Extension for Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154, March 2011.
[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017.

8.2. Informative References

[RFC3348] Gahrns, M. and R. Cheng, "The Internet Message Action Protocol (IMAP4) Child Mailbox Extension", RFC 3348, DOI 10.17487/RFC3348, July 2002.
[RFC5788] Melnikov, A. and D. Cridland, "IMAP4 Keyword Registry", RFC 5788, DOI 10.17487/RFC5788, March 2010.

Author's Address

Barry Leiba (editor) Huawei Technologies Phone: +1 646 827 0648 EMail: URI: