Internet Engineering Task Force (IETF)                  B. Gondwana, Ed.
Request for Comments: 9042                                      Fastmail
Updates: 5228                                                  June 2021
Category: Standards Track
ISSN: 2070-1721


             Sieve Email Filtering: Delivery by MAILBOXID

Abstract

  The OBJECTID capability of IMAP (RFC 8474) allows clients to identify
  mailboxes by a unique identifier that survives renaming.

  This document extends the Sieve email filtering language (RFC 5228)
  to allow using that same unique identifier as a target for fileinto
  rules and for testing the existence of mailboxes.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9042.

Copyright Notice

  Copyright (c) 2021 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
  (https://trustee.ietf.org/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.  Conventions Used in This Document
  3.  Sieve Capability String
  4.  Argument :mailboxid to Command fileinto
    4.1.  Interaction with Mailbox Extension
    4.2.  Interaction with Special-Use Extension
  5.  Interaction with FCC Extension
  6.  Test mailboxidexists
  7.  Interaction with Variables Extension
  8.  Security Considerations
  9.  IANA Considerations
  10. References
    10.1.  Normative References
    10.2.  Informative References
  Acknowledgements
  Author's Address

1.  Introduction

  Sieve rules [RFC5228] are sometimes created using graphical
  interfaces, which allow users to select the mailbox to be used as a
  target for a rule.

  If that mailbox is renamed, the client may also update its internal
  representation of the rule and update the Sieve script to match;
  however, this is a multistep process and subject to partial failures.
  Also, if the folder is renamed by a different mechanism (e.g.,
  another IMAP client), the rules will get out of sync.

  By telling fileinto to reference the immutable MAILBOXID specified by
  [RFC8474], using the extension specified herein, Sieve rules can
  continue to target the same mailbox, even if it gets renamed.

2.  Conventions Used in This Document

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.

3.  Sieve Capability String

  Scripts that use the extensions defined in this document MUST
  explicitly require the capability "mailboxid".

  Example:

  require "mailboxid";

4.  Argument :mailboxid to Command fileinto

  Normally, the fileinto command delivers the message in the mailbox
  specified using its positional mailbox argument.  However, if the
  optional :mailboxid argument is also specified, the fileinto command
  first checks whether a mailbox exists in the user's personal
  namespace [RFC2342] with the specified MAILBOXID [RFC8474].

  If a matching mailbox is found, that mailbox is used for delivery.

  If there is no such mailbox, the fileinto action proceeds as it would
  without the :mailboxid argument.

  The tagged argument :mailboxid to fileinto consumes one additional
  token, a string containing the OBJECTID of the target mailbox.

  Example:

  require "fileinto";
  require "mailboxid";

  if header :contains ["from"] "coyote" {
      fileinto :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3"
               "INBOX.harassment";
  }

4.1.  Interaction with Mailbox Extension

  For servers that also support the mailbox extension defined in
  [RFC5490], if both the :create and :mailboxid arguments are provided
  to a fileinto command and no matching mailbox is found, then a new
  mailbox will be created.

  This new mailbox will have the name specified by the positional
  mailbox argument ([RFC5228], Section 4.1); however, it will get a
  different MAILBOXID (chosen by the server) rather than the one
  specified by the :mailboxid argument to fileinto.

  Example:

  require "fileinto";
  require "mailboxid";
  require "mailbox";

  fileinto :mailboxid "Fnosuch"
           :create
           "INBOX.no-such-folder";
              # creates INBOX.no-such-folder, but it doesn't
              # get the "Fnosuch" mailboxid.

4.2.  Interaction with Special-Use Extension

  For servers that also support delivery to special-use mailboxes
  [RFC8579], it is an error to specify both :mailboxid and :specialuse
  in the same fileinto command.

  Advanced filtering based on both special-use and MAILBOXID can be
  built with explicit specialuse_exists and mailboxidexists tests.

     |  Note to developers of Sieve generation tools:
     |
     |  It is advisable to use special-use rather than MAILBOXID when
     |  creating rules that are based on a special-use purpose (e.g.,
     |  delivery directly to the Junk folder based on a header that was
     |  added by a scanning agent earlier in the mail flow).

5.  Interaction with FCC Extension

  This document extends the definition of the :fcc argument defined in
  [RFC8580] so that it can optionally be used with the :mailboxid
  argument.  The syntax for FCC is extended here using ABNF [RFC5234]:

  MAILBOXID-OPT = ":mailboxid" objectid

  FCC-OPTS =/ MAILBOXID-OPT

  If the optional :mailboxid argument is specified with :fcc, it
  instructs the Sieve interpreter to check whether a mailbox exists
  with the specific MAILBOXID.  If such a mailbox exists, the generated
  message is filed into that mailbox.  Otherwise, the generated message
  is filed into the :fcc target mailbox.

  As with fileinto, it is an error to specify both :mailboxid and
  :specialuse for the same fcc rule.

  Example:

  require ["enotify", "fcc", "mailboxid"];
  notify :fcc "INBOX.Sent"
         :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3"
         :message "You got mail!"
         "mailto:[email protected]";

6.  Test mailboxidexists

  Usage: mailboxidexists <mailbox-objectids: string-list>

  The mailboxidexists test is true if every string argument provided is
  the MAILBOXID of a mailbox that exists in the mailstore and that
  allows the user in whose context the Sieve script runs to deliver
  messages into it.

  When the mailstore is an IMAP server that also supports IMAP Access
  Control List (ACL) [RFC4314], delivery is allowed if the user has the
  'p' or 'i' rights for the mailbox (see Section 5.2 of [RFC4314]).

  When the mailstore is an IMAP server that does not support IMAP ACL,
  delivery is allowed if the READ-WRITE response code is present for
  the mailbox when selected by the user (see Section 7.1 of [RFC3501]).

  Note that a successful mailboxidexists test for a mailbox doesn't
  necessarily mean that a "fileinto :mailboxid" action on this mailbox
  would succeed.  For example, the fileinto action might put the user
  over quota.  The mailboxidexists test only verifies existence of the
  mailbox and whether the user in whose context the Sieve script runs
  has permissions to execute fileinto on it.

  Example:

  require "fileinto";
  require "mailboxid";

  if header :contains ["from"] "coyote" {
      if mailboxidexists "F6352ae03-b7f5-463c-896f-d8b48ee3" {
          fileinto :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3"
                              "INBOX.name.will.not.be.used";
      } else {
          fileinto "INBOX.harassment";
      }
  }

     |  Note to implementers:
     |
     |  This test behaves identically to the mailboxexists test defined
     |  in [RFC5490] but operates on MAILBOXIDs rather than mailbox
     |  names.

7.  Interaction with Variables Extension

  There is no special interaction defined; however, as an OBJECTID is a
  string in this document, OBJECTID values can contain variable
  expansions if [RFC5229] is enabled.

8.  Security Considerations

  Because MAILBOXID is always generated by the server, implementations
  MUST NOT allow Sieve to make an end run around this protection by
  creating mailboxes with the specified ID by using :create and
  :mailboxid in a fileinto rule for a nonexistent mailbox.

  Implementers are referred to the Security Considerations sections of
  [RFC5228] and [RFC8474].

9.  IANA Considerations

  IANA has added the following capability to the "Sieve Extensions"
  registry at <https://www.iana.org/assignments/sieve-extensions>:

  Capability name:  mailboxid
  Description:  adds a test for checking mailbox existence by OBJECTID
     and new optional arguments to fileinto and :fcc that allow
     selecting the destination mailbox by OBJECTID.
  RFC number:  RFC 9042
  Contact address:  EXTRA discussion list <[email protected]>

10.  References

10.1.  Normative References

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <https://www.rfc-editor.org/info/rfc2119>.

  [RFC2342]  Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
             DOI 10.17487/RFC2342, May 1998,
             <https://www.rfc-editor.org/info/rfc2342>.

  [RFC5228]  Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
             Filtering Language", RFC 5228, DOI 10.17487/RFC5228,
             January 2008, <https://www.rfc-editor.org/info/rfc5228>.

  [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", STD 68, RFC 5234,
             DOI 10.17487/RFC5234, January 2008,
             <https://www.rfc-editor.org/info/rfc5234>.

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  [RFC8474]  Gondwana, B., Ed., "IMAP Extension for Object
             Identifiers", RFC 8474, DOI 10.17487/RFC8474, September
             2018, <https://www.rfc-editor.org/info/rfc8474>.

  [RFC8580]  Murchison, K. and B. Gondwana, "Sieve Extension: File
             Carbon Copy (FCC)", RFC 8580, DOI 10.17487/RFC8580, May
             2019, <https://www.rfc-editor.org/info/rfc8580>.

10.2.  Informative References

  [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
             4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
             <https://www.rfc-editor.org/info/rfc3501>.

  [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
             RFC 4314, DOI 10.17487/RFC4314, December 2005,
             <https://www.rfc-editor.org/info/rfc4314>.

  [RFC5229]  Homme, K., "Sieve Email Filtering: Variables Extension",
             RFC 5229, DOI 10.17487/RFC5229, January 2008,
             <https://www.rfc-editor.org/info/rfc5229>.

  [RFC5490]  Melnikov, A., "The Sieve Mail-Filtering Language --
             Extensions for Checking Mailbox Status and Accessing
             Mailbox Metadata", RFC 5490, DOI 10.17487/RFC5490, March
             2009, <https://www.rfc-editor.org/info/rfc5490>.

  [RFC8579]  Bosch, S., "Sieve Email Filtering: Delivering to Special-
             Use Mailboxes", RFC 8579, DOI 10.17487/RFC8579, May 2019,
             <https://www.rfc-editor.org/info/rfc8579>.

Acknowledgements

  This document borrows heavily from [RFC5490] for the matching
  mailboxexists test and from [RFC8579] for an example of modifying the
  fileinto command.

  Thanks to Ned Freed, Ken Murchison, and Alexey Melnikov for feedback
  on the EXTRA mailing list.

Author's Address

  Bron Gondwana (editor)
  Fastmail
  Level 2
  114 William St
  Melbourne VIC 3000
  Australia

  Email: [email protected]
  URI:   https://www.fastmail.com

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