Sieve Email Filtering: Delivering to Special-Use Mailboxes
draft-ietf-extra-sieve-special-use-03

EXTRA                                                           S. Bosch
Internet-Draft                                                Dovecot Oy
Intended status: Standards Track                       September 5, 2018
Expires: March 9, 2019

       Sieve Email Filtering: Delivering to Special-Use Mailboxes
                 draft-ietf-extra-sieve-special-use-03

Abstract

   The SPECIAL-USE capability of the IMAP protocol (RFC 6154) allows
   clients to identify special-use mailboxes; e.g., where draft or sent
   messages should be put.  This simplifies client configuration.  In
   contrast, the Sieve mail filtering language (RFC 5228) currently has
   no such capability.  This memo defines a Sieve extension that fills
   this gap: it adds a test for checking whether a special-use attribute
   is assigned for a particular mailbox or any mailbox, and it adds the
   ability to file messages into an anonymous mailbox that has a
   particular special-use attribute assigned.

Status of This Memo

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   This Internet-Draft will expire on March 9, 2019.

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Test "specialuse_exists"  . . . . . . . . . . . . . . . . . .   3
   4.  ":specialuse" Argument to "fileinto" Command  . . . . . . . .   4
     4.1.  Mailboxes Created Implicitly by the "fileinto" Command  .   5
   5.  Sieve Capability Strings  . . . . . . . . . . . . . . . . . .   6
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Commonly, several mailboxes in an IMAP message store [IMAP] have a
   special use; e.g.  it is where the user's draft messages are stored,
   where a copy of sent messages are kept, or it is where spam messages
   are filed automatically at delivery.  The SPECIAL-USE capability
   [SPECIAL-USE] of the IMAP protocol defines mailbox attributes that
   identify these special mailboxes explicitly to the client.  This way,
   client configuration is simplified significantly.  Using the CREATE-
   SPECIAL-USE capability [SPECIAL-USE], IMAP clients can also configure
   these attributes dynamically based on user preference.

   Unlike the IMAP protocol, the Sieve mail filtering language [SIEVE]
   currently cannot freely access these special-use mailbox attributes.
   Particularly, the Sieve interpreter cannot find an anonymous mailbox
   that has a particular special-use attribute assigned.  This would be
   very useful for example to find the user's Spam mailbox at delivery.

   In Sieve, limited access to the special-use attributes is provided
   using the "mboxmetadata" extension [SIEVE-MAILBOX], which allows
   testing for the presence of a special-use attribute in the "/private/
   specialuse" IMAP METADATA [IMAP-METADATA] entry of a mailbox.  Still,
   not all implementers will be willing to add the complexity of the
   IMAP METADATA capability, just to provide access to special-use
   attributes to the Sieve interpreter.

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   This document defines an extension to the Sieve mail filtering
   language that adds the ability to freely access mailbox special-use
   attributes.  It adds a test called "specialuse_exists" that checks
   whether a special-use attribute is assigned for a particular mailbox
   or - if omitted - any of the user's personal mailboxes.  It also adds
   the ability to file messages into an anonymous personal mailbox that
   has a particular special-use attribute assigned using a ":specialuse"
   argument for the "fileinto" command [SIEVE].

2.  Conventions Used in This Document

   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 [KEYWORDS].

   Conventions for notations are as in [SIEVE] Section 1.1, including
   use of the "Usage:" label for the definition of action and tagged
   arguments syntax.

3.  Test "specialuse_exists"

   Usage:  specialuse_exists [<mailbox: string>]
                             <special-use-flags: string-list>

   If the "mailbox" string argument is omitted, the "specialuse_exists"
   test yields true if all of the following statements are true for each
   of the special-use flags listed in the "special-use-flags" argument:

   a.  at least one mailbox exists in the user's personal namespace
       [NAMESPACE] that has that particular special-use flag assigned,
       and

   b.  that mailbox allows the user in whose context the Sieve script
       runs to "deliver" messages into it.

   If the "mailbox" argument is specified, the "specialuse_exists" test
   yields true if all of the following statements are true:

   a.  the indicated mailbox exists,

   b.  that mailbox allows the user in whose context the Sieve script
       runs to "deliver" messages into it, and

   c.  that mailbox has all of the special-use flags listed in the
       "special-use-flags" argument assigned to it.

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   Refer to the specification of the "mailboxexists" test in Section 3.1
   of RFC 5490 [SIEVE-MAILBOX] for a definition of when "delivery" of
   messages into a mailbox is deemed possible.

4.  ":specialuse" Argument to "fileinto" Command

   Usage:  fileinto [:specialuse <special-use-flag: string>]
                    <mailbox: string>

   Normally, the "fileinto" command delivers the message in the mailbox
   specified using its positional mailbox argument.  However, if the
   optional ":specialuse" argument is also specified, the "fileinto"
   command first checks whether a mailbox exists in the user's personal
   namespace [NAMESPACE] with the specified special-use flag assigned to
   it.  If that is the case, that special-use mailbox is used for
   delivery instead.  If there is no such mailbox or if the specified
   special-use flag is unknown to the implementation in general, the
   "fileinto" action proceeds as it would without the ":specialuse"
   argument.

   Summarizing, if the ":specialuse" argument is specified, the fileinto
   command deals with two mailboxes that may or may not exist:

   o  An anonymous special-use mailbox in the user's personal namespace,
      which has at least the special-use flag specified with the
      ":specialuse" argument assigned to it.

   o  The default mailbox named by the positional string argument of the
      "fileinto" command, which is used when the special-use mailbox is
      not found.

   The special-use flag specified with the ":specialuse" argument MUST
   conform to the "use-attr" syntax described in Section 6 of RFC6154
   [SIEVE-MAILBOX].  Implementations SHOULD handle an invalid special-
   use flag in the same way as an invalid mailbox name is handled.  The
   string parameter of the ":specialuse" argument is not a constant
   string, which means that variable substitutions are allowed when the
   "variables" extension [VARIABLES] is active.  In that case, the
   syntax of the special-use flag is only verified at runtime.

   If neither the special-use mailbox nor the default mailbox exists,
   the "fileinto" action MUST proceed exactly as it does in case the
   ":specialuse" is argument is absent and the mailbox named by its
   positional argument does not exist.  The various options for handling
   this situation are described in Section 4.1 of RFC5228 [SIEVE].

   More than one mailbox in the user's personal namespace can have a
   particular special-use flag assigned.  If one of those mailboxes is

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   in fact the default mailbox named by the positional string argument
   of the "fileinto" command, that mailbox MUST be used for delivery.
   If the default mailbox is not one of the options, the mailbox that is
   chosen for delivery is implementation-defined.  However, while the
   set of mailboxes to which the involved special-use flags are assigned
   remains unchanged, implementations SHOULD ensure that the mailbox
   choice is made consistently, so that the same mailbox is used every
   time.  Conversely, the chosen mailbox MAY change once the special-use
   flag assignments that are relevant for the mailbox choice are changed
   (usually by user interaction).

   If delivery to the special-use mailbox fails for reasons not relating
   to its existence, the Sieve interpreter MUST NOT subsequently attempt
   delivery in the indicated default mailbox as a fall-back.  Instead,
   it MUST proceed exactly as it does in case the ":specialuse" argument
   is absent and delivery to the mailbox named by its positional
   argument fails.  This prevents the situation where messages are
   unexpectedly spread over two mailboxes in case transient or
   intermittent delivery failures occur.

4.1.  Mailboxes Created Implicitly by the "fileinto" Command

   Before attempting to deliver the message into the specified mailbox,
   the "fileinto" command may implicitly create the mailbox if it does
   not exist (see Section 4.1 of RFC5228 [SIEVE]).  This optional
   behavior can be requested explicitly using the "mailbox" extension
   [SIEVE-MAILBOX], which adds the optional ":create" argument to the
   "fileinto" command.  If the ":create" argument is specified with
   "fileinto", it instructs the Sieve interpreter to unconditionally
   create the specified mailbox if needed.  Note that the ":create"
   argument has no effect when the implicit creation of mailboxes for
   delivery is the default behavior.

   When the ":specialuse" argument is present, this behavior does not
   change: the Sieve interpreter will implicitly create the specified
   default mailbox if needed.  This need arises when both the special-
   use mailbox and the default mailbox are not found.

   If the server implementation supports the CREATE-SPECIAL-USE
   capability [SPECIAL-USE] for IMAP (i.e., it allows assigning special-
   use flags to new mailboxes) it SHOULD assign the special-use flag
   specified with the ":specialuse" argument to the newly created
   mailbox.

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5.  Sieve Capability Strings

   A Sieve implementation that defines the "specialuse_exists" test and
   the ":specialuse" argument for the "fileinto" command will advertise
   the capability string "special-use".

6.  Examples

   The following example saves the message in the mailbox where messages
   deemed to be junk mail are held.  This mailbox is identified using
   the "\Junk" special-use attribute.  If no mailbox has this attribute
   assigned, the message is filed into the mailbox named "Spam".  If the
   mailbox named "Spam" does not exist either, the result of this Sieve
   script is implementation-dependent: e.g., it may trigger an error or
   the mailbox may be created implicitly.

   require "fileinto";
   require "special-use";

   fileinto :specialuse "\\Junk" "Spam";

   The following very similar example explicitly handles the case in
   which neither a "\Junk" special-use mailbox nor the "Spam" mailbox
   exist.  In that case, a mailbox called "Spam" is created, and the
   message is stored there.  Additionally, the "\Junk" special-use
   attribute may be assigned to it.

   require "fileinto";
   require "special-use";
   require "mailbox";

   fileinto :specialuse "\\Junk" :create "Spam";

   The following example is used in a Sieve script that is triggered
   from an IMAP event, rather than at message delivery [IMAPSIEVE].
   This Sieve script redirects messages to an automated recipient that
   processes junk mail, if those messages are copied or moved into a
   mailbox that has the "\Junk" special-use attribute assigned.

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   require "imapsieve";
   require "special-use";
   require "environment";
   require "variables";

   if environment :contains "imap.mailbox" "*" {
       set "mailbox" "${1}";
   }

   if allof(
       environment "imap.cause" "COPY",
       specialuse_exists "${mailbox}" "\\Junk") {
       redirect "spam-report@example.org";
   }

7.  Security Considerations

   Security considerations are discussed in [SIEVE], [VARIABLES], and
   [SPECIAL-USE].  It is believed that this extension does not introduce
   any additional security concerns.

   Note that this specification explicitly restricts the anonymous
   special-use mailbox to the user's personal namespace.  First, this
   avoids the need to search the entire mail storage for mailboxes that
   have a particular special-use flag assigned.  This could put undue
   load on the system, while shared special-use mailboxes are deemed of
   limited use with the currently defined special-use flags.  Secondly,
   it prevents security concerns with shared mailboxes that have
   special-use flags assigned that apply to all users.  Searching the
   entire mail storage for special-use mailboxes could lead to messages
   unexpectedly or even maliciously being filed to shared mailboxes.

   This restriction could be lifted for particular future special-use
   flags, but such new flags should have a clear application for shared
   mailboxes and the security concerns should be considered carefully.

8.  IANA Considerations

   The following template specifies the IANA registration of the Sieve
   extension specified in this document:

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      To: iana@iana.org
      Subject: Registration of new Sieve extension

      Capability name: special-use
      Description:     adds a test for checking whether an IMAP
                       special-use attribute is assigned for a
                       particular mailbox or any mailbox, and it adds
                       the ability to file messages into an anonymous
                       mailbox that has a particular IMAP special-use
                       attribute assigned.
      RFC number:      this RFC
      Contact address: Sieve mailing list <sieve@ietf.org>

   This information should be added to the list of sieve extensions
   given on http://www.iana.org/assignments/sieve-extensions.

9.  Acknowledgements

   Thanks to Stan Kalisch, Barry Leiba, Alexey Melnikov, Ken Murchison,
   and Ned Freed for reviews and suggestions.

   Thanks to the authors of RFC5490 [SIEVE-MAILBOX] from which some
   descriptive text is borrowed in this document.

10.  References

10.1.  Normative References

   [IMAP-METADATA]
              Daboo, C., "The IMAP METADATA Extension", RFC 5464,
              DOI 10.17487/RFC5464, February 2009,
              <http://www.rfc-editor.org/info/rfc5464>.

   [KEYWORDS]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [SIEVE]    Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

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   [SIEVE-MAILBOX]
              Melnikov, A., "The Sieve Mail-Filtering Language --
              Extensions for Checking Mailbox Status and Accessing
              Mailbox Metadata", RFC 5490, March 2009.

   [SPECIAL-USE]
              Leiba, B. and J. Nicolson, "IMAP LIST Extension for
              Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154,
              March 2011, <http://www.rfc-editor.org/info/rfc6154>.

   [VARIABLES]
              Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.

10.2.  Informative References

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

   [IMAPSIEVE]
              Leiba, B., "Support for Internet Message Access Protocol
              (IMAP) Events in Sieve", RFC 6785, DOI 10.17487/RFC6785,
              November 2012, <http://www.rfc-editor.org/info/rfc6785>.

Author's Address

   Stephan Bosch
   Dovecot Oy
   Lars Sonckin Kaari 12
   Espoo  02600
   Finland

   Email: stephan.bosch@dovecot.fi

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