Network Working Group                                      B. Leiba, Ed.
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                            May 01,                           June 07, 2018
Expires: October 31, December 07, 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, 5258, 3501,
   and 6154.  This document also defines a new IMAP keyword,
   "$Important", and registers it in the registry defined in RFC 5788.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 31, December 07, 2018.

Copyright Notice

   Copyright (c) 2018 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
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   1.1.  Conventions used in this document  . . . . . . . . . . . . .  2
   2.  Definition of the '$Important' Message Keyword . . . . . . . .  2
   3.  Definition of the 'Important' Mailbox Attribute  . . . . . . .  3
   3.1.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2.1.  Example of a LIST Response . . . . . . . . . . . . . . . .  3
   3.2.2.  Examples of Creating a New Mailbox using \Important  . . .  4
   4.  Implementation Notes . . . . . . . . . . . . . . . . . . . . .  4
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   6.1.  Registration of the $Important keyword . . . . . . . . . . .  5
   6.2.  Creation of the IMAP Mailbox Name Attributes Registry  . . .  6
   6.2.1.  Instructions to the Designated Expert  . . . . . . . . . .  7
   6.3.  Initial Entries for the IMAP Mailbox Name Attributes Registry 7
   7.  Changes During Document Development  . . . . . . . . . . . . .  8
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . .  9
   9.2.  Informative References . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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:

   o  Defines a new message keyword, "$Important", to apply to messages
      that are considered important for the user, by some externally
      defined criteria.

   o  Registers the "$Important" keyword in the IMAP Keywords registry.

   o  Defines a new special-use attribute, "\Important", to designate a
      mailbox that will hold messages that are considered important for
      the user, by some externally defined criteria.

   o  Creates a registry for IMAP mailbox attributes and registers the
      new attribute and those defined in [RFC3348], [RFC5258], [RFC3501], and

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 is generally expected to be set
   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).  While the keyword
   also can be set by the user, that is not expected to be the primary

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

   1.  "$Important" carries a meaning of general importance, as opposed
       to follow-up or urgency.  It is meant to be used for a form of
       triage, with "\Flagged" remaining as a designation of special
       attention, need for follow-up, or time-sensitivity.  In
       particular, the sense of "$Important" is that other messages that
       are "like this one" according to some server-applied heuristics
       will also be $Important.

   2.  The setting of "$Important" is expected to be based at least
       partly on heuristics, generally set automatically by the server,
       whereas "\Flagged" is only intended to be set by the user with
       some sort of "flag this message" or "put a star on this message"

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

   The following syntax specification adds to the one in [RFC6154],
   Section 6, using Augmented Backus-Naur Form (ABNF) as described in
   [RFC5234].  Be sure to see the ABNF notes at the beginning of
   [RFC3501], Section 9.

    use-attr      =/  "\Important"

3.2.  Examples

3.2.1.  Example of a LIST Response

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

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

3.2.2.  Examples of Creating a New Mailbox using \Important

   In the following example, the mailbox called "Important Messages" is
   created with the "\Important" attribute on a server that advertises
   the "CREATE-SPECIAL-USE" capability string.

    C: t1 CREATE "Important Messages" (USE (\Important))
    S: t1 OK Mailbox created

   The following example is similar to the previous one, but the server
   is not able to assign the \Important attribute to the new mailbox.

    C: t1 CREATE "Important Messages" (USE (\Important))
    S: t1 NO [USEATTR] Mailbox not Not created; an \Important mailbox already exists

   The following example is similar to the previous one, but the server
   does not support this extension.

    C: t1 CREATE "Important Messages" (USE (\Important))
    S: t1 NO [USEATTR] Mailbox not created; unsupported use \Important

   In both of the failure-mode examples, the "USEATTR" response code
   lets the client know that the problem is in the "USE" parameters.
   Note that the same response code is given in both cases, and the
   human-readable text is the only way to tell the difference.  That
   text is not parsable by the client (it can only be logged and/or
   reported to the user).

4.  Implementation Notes

   This section is non-normative and is intended to describe the
   intended (and current as of this publication) usage of "$Important"
   in contrast with "\Flagged" on a message.

   On the server:

   o  \Flagged is set or cleared in response to an explicit command from
      the client.

   o  $Important is set via a heuristic process performed by the server,
      usually involving analysis of header fields, what mailbox the
      message is filed in, perhaps message content, attachments, and
      such.  It may then be set or cleared in response to an explicit
      command from the client, and the server may use that to adjust the
      heuristics in the future.  It's also possible that the server will
      re-evaluate this and make a message $Important later if the user
      accesses the message frequently, for example.

   On the client:

   o  Typically, an icon such as a flag or a star, or an indication such
      as red or bold text, is associated with \Flagged, and the UI
      provides a way for the user to turn that icon or indication on or
      off.  Manipulation of the this results in a command to the server.

   o  Typically, a lesser indication is used for $Important.  The client
      might or might not provide the user with a way to manipulate it.
      If it does, manipulation results in a command to the server.

5.  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

   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.  See [RFC3501], Section 11, for security
   considerations about using the IMAP STARTTLS command to protect the
   IMAP channel.

6.  IANA Considerations

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

   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.

6.1.  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: $Important
   Purpose (description): The "$Important" keyword is a signal that a
         message is likely important to the user.

   Private or Shared on a server: PRIVATE

   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

   Related keywords: None (but see the reference for the related mailbox
         name attribute).

   Related IMAP capabilities: None.

   Security considerations: See [[THIS RFC]], Section 5

   Published specification: [[THIS RFC]]

   Person & email address to contact for further information:
         IETF Applications and Real-Time Area <>

   Intended usage: COMMON

   Owner/Change controller: IESG

   Note: None.

6.2.  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 6".  This registry will be
   shared with the JSON Meta Application Protocol (JMAP) for Mail [I-D

   The registry entries will contain the following fields:

   1.  Attribute Name
   2.  Description
   3.  Reference
   4.  Usage Notes

   IANA will keep this list in alphabetical order by Attribute Name,
   which is registered without the initial backslash ("\").  The names
   are generally registered with initial capital letters, but are
   treated as case-insensitive US-ASCII strings.

   The "Usage Notes" field is free-form US-ASCII text that will normally
   be empty (and is empty if it's not specified in the registration
   request).  It is intended to hold things such as "not used by JMAP"
   and "JMAP only".  The field is for human use, and there is no need
   for a registry of strings that may appear here.

   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 fields.

   2.  For other registrations, the policy will be Expert Review policy
       (see Section 6.2.1), 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 fields.  While a
       formal specification is not required, the reference document
       should provide a description of the proposed attribute sufficient
       for building interoperable implementations.  An Informational RFC
       (submitted through the IETF or Independent stream) is a fine way
       to publish a reference document (see also Section 6.2.1).

6.2.1.  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.

6.3.  Initial Entries for the IMAP Mailbox Name Attributes Registry

   The registry will initially contain these entries:

   | 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    | [RFC5258] | *
   | HasNoChildren | Has no accessible child mailboxes | [RFC5258] | *
   | 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] | *
   | NonExistent   | The mailbox name doesn't actually | [RFC5258] | *
   |               | exist                             |           |
   | Noselect      | The mailbox is not selectable     | [RFC3501] | *
   | Remote        | The mailbox exists on a remote    | [RFC5258] | *
   |               | server                            |           |
   | Sent          | Sent mail                         | [RFC6154] |
   | Subscribed    | The mailbox is subscribed to      | [RFC5258] |
   | Trash         | Messages the user has discarded   | [RFC6154] |
   | Unmarked      | No new messages since last select | [RFC3501] | *

   The rows marked with "*" at the end should have their Usage Notes
   field set to "not used by JMAP".

7.  Changes During Document Development

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

   Changes in draft-ietf-extra-specialuse-important-00

   o  Removed "specific" from "a specific meaning of general importance"
      because it sounded stupid.

   o  Added a "Usage Notes" column to the registry table in 6.2, and
      called out some "not used by JMAP" in 6.3.

   Changes in draft-leiba-extra-specialuse-important-01

   o  Updated "IETF Applications Area" to "IETF Applications and Real-
      Time Area".

   o  Changed some wording to make the distinction between \Flagged and
      \Important clearer.

   o  Added some text explaining how \Important is used in existing

   o  Added a note in the ABNF section referring to the ABNF notes in
      the IMAP spec.

   Changes in draft-leiba-extra-specialuse-important-00

   o  Reset status, moved Eric to "Contributors", changed Barry to

   o  Updated BCP 26 reference to RFC 8126.

   Changes in draft-iceman-imap-specialuse-important-02

   o  Added the definition and registration of $Important.

   o  Noted that \Important might be implemented as a virtual collection
      of $Important messages.

   Changes in draft-iceman-imap-specialuse-important-01

   o  Expanded the new registry to all mailbox name attributes, and
      added the attributes from 3501 and 3348 5258 (suggested by Alexey).
      This also adds those two documents to the "updates" list.

   o  Recorded Cyrus's suggestion to define $Important.

8.  Contributors

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

   Eric "Iceman"

9.  References

9.1.  Normative References

              4rev1", RFC 3501, March 2003.

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

   [RFC6154]  Leiba, B. and J. Nicolson, "IMAP LIST Extension for
              Special-Use Mailboxes", RFC 6154, 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, <https://www

9.2.  Informative References

              Jenkins, N., "JMAP for Mail", Internet-Draft draft-ietf-
              jmap-mail-04, March 2018.

   [RFC3348]  Gahrns, M. and R. Cheng, "The Internet

   [RFC5258]  Leiba, B. and A. Melnikov, "Internet Message Action Access
              Protocol (IMAP4) Child Mailbox Extension", version 4 - LIST Command Extensions", RFC 3348, July
              2002. 5258,
              DOI 10.17487/RFC5258, June 2008, <https://www.rfc-

   [RFC5788]  Melnikov, A. and D. Cridland, "IMAP4 Keyword Registry",
              RFC 5788, March 2010.

Author's Address

   Barry Leiba, editor
   Huawei Technologies

   Phone: +1 646 827 0648