    Internet Engineering Task Force (IETF)                      K. Murchison
    Request for Comments: 8580                                   B. Gondwana
    Updates: 5230, 5435                                             FastMail
    Category: Standards Track                                       May 2019
    ISSN: 2070-1721

                    Sieve Extension: File Carbon Copy (FCC)

    Abstract

       The Sieve email filtering language provides a number of action
       commands, some of which can generate additional messages on behalf of
       the user.  This document defines an extension to such commands to
       allow a copy of any generated message to be filed into a target
       mailbox.

       This document updates RFCs 5230 and 5435 by adding a new tagged
       argument to the Vacation and Notify actions, respectively.

    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/rfc8580.

    Copyright Notice

       Copyright (c) 2019 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.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
       2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
       3.  Tagged Argument ":fcc"  . . . . . . . . . . . . . . . . . . .   3
         3.1.  Interaction with Extensions to the Fileinto Action  . . .   3
           3.1.1.  Imap4flags Extension  . . . . . . . . . . . . . . . .   4
           3.1.2.  Mailbox Extension . . . . . . . . . . . . . . . . . .   4
           3.1.3.  Special-Use Extension . . . . . . . . . . . . . . . .   4
         3.2.  Collected Grammar . . . . . . . . . . . . . . . . . . . .   5
       4.  Format of FCC Messages  . . . . . . . . . . . . . . . . . . .   5
       5.  Interaction with the Vacation Action  . . . . . . . . . . . .   6
       6.  Interaction with the Notify Action  . . . . . . . . . . . . .   7
         6.1.  Notification-Capability "fcc" . . . . . . . . . . . . . .   7
       7.  Compatibility with the Reject and Extended Reject
           Actions . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       8.  Compatibility with Other Actions  . . . . . . . . . . . . . .   8
       9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
       10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
       11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
         11.1.  Registration of New Sieve Extension  . . . . . . . . . .   9
         11.2.  Registration of New Notification-Capability
                Parameter  . . . . . . . . . . . . . . . . . . . . . . .  10
       12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
         12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
         12.2.  Informative References . . . . . . . . . . . . . . . . .  12
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

    1.  Introduction

       The Sieve email filtering language [RFC5228] provides a number of
       action commands, some of which can generate additional messages on
       behalf of the user.  It is sometimes desirable for a Sieve user to
       maintain an archive of the messages generated by these commands.

       This extension defines ":fcc", a new optional tagged argument for
       action commands that generate additional messages.  This argument
       allows a copy of the generated message to be filed into a target
       mailbox.

       The capability string associated with this extension is "fcc".

       Each new action that generates additional messages will need to
       specify how it interacts with the FCC extension.  This document
       specifies the interaction of the FCC extension with the Vacation
       [RFC5230] and Notify [RFC5435] actions.

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    2.  Conventions Used in This Document

       Conventions for notations are as described in Section 1.1 of
       [RFC5228], including use of the "Usage:" label for the definition of
       the action and the syntax of tagged arguments.

       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.  Tagged Argument ":fcc"

       This document specifies ":fcc", a new optional tagged argument that
       alters the behavior of action commands that generate additional
       messages on behalf of the user.

       Usage: :fcc <mailbox: string>

       The ":fcc" tagged argument instructs the Sieve interpreter to file a
       copy of the generated message into the mailbox provided in the
       subsequent argument.  The semantics and treatment of the mailbox
       argument are defined to match those of the mailbox argument to the
       fileinto action specified in Section 4.1 of [RFC5228].  Specifically,
       use of an invalid mailbox name MAY be treated as an error or result
       in delivery to an implementation-defined mailbox, and if the
       specified mailbox doesn't exist, the implementation MAY treat it as
       an error, create the mailbox, or file the message into an
       implementation-defined mailbox.

    3.1.  Interaction with Extensions to the Fileinto Action

       Some tagged arguments defined in extensions to the fileinto action
       can be used together with the ":fcc" tagged argument.  The sections
       below describe these interactions.  Tagged arguments in future
       extensions to the fileinto action need to describe their interaction
       with the FCC extension, if any.

       When any fileinto extension arguments are used with the FCC
       extension, the corresponding extension MUST be enabled, and the
       arguments are defined to have the same syntax, semantics, and
       treatment as they do with the fileinto action.

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    3.1.1.  Imap4flags Extension

       This document extends the definition of the ":flags" tagged argument
       (see Section 5 of [RFC5232]) so that it can optionally be used with
       the ":fcc" tagged argument.

       Usage: :fcc <mailbox: string> [:flags <list-of-flags: string-list>]

       If the optional ":flags" tagged argument is specified with the ":fcc"
       tagged argument, it instructs the Sieve interpreter to set the IMAP4
       flags provided in the subsequent argument when the generated message
       is filed into the target mailbox.

    3.1.2.  Mailbox Extension

       This document extends the definition of the ":create" tagged argument
       (see Section 3.2 of [RFC5490]) so that it can optionally be used with
       the ":fcc" tagged argument.

       Usage: :fcc <mailbox: string> [:create]

       If the optional ":create" tagged argument is specified with the
       ":fcc" tagged argument, it instructs the Sieve interpreter to create
       the target mailbox, if needed, before attempting to file the
       generated message into the target mailbox.

    3.1.3.  Special-Use Extension

       This document extends the definition of the ":specialuse" tagged
       argument (see Section 4 of [RFC8579]) so that it can optionally be
       used with the ":fcc" tagged argument.

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

       If the optional ":specialuse" tagged argument is specified with the
       ":fcc" tagged argument, it instructs the Sieve interpreter to check
       whether a mailbox exists with the specific special-use flag assigned
       to it.  If such a mailbox exists, the generated message is filed into
       the special-use mailbox.  Otherwise, the generated message is filed
       into the target mailbox.

       If the optional ":specialuse" and ":create" tagged arguments are both
       specified with the ":fcc" tagged argument, the Sieve interpreter is
       instructed to create the target mailbox per Section 4.1 of [RFC8579],
       if needed.

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    3.2.  Collected Grammar

       For convenience, the "FCC" syntax element is defined here using ABNF
       [RFC5234] so that it can be augmented by other extensions.

       Note that the following is the grammar of "FCC" after it has been
       lexically interpreted.  No whitespace or comments appear below.

       FCC         = ":fcc" string *FCC-OPTS
                       ; per Section 2.6.2 of RFC 5228,
                       ; the tagged arguments in FCC may appear in any order

       FCC-OPTS    = CREATE / IMAP-FLAGS / SPECIAL-USE
                       ; each option MUST NOT appear more than once

       CREATE      = ":create"
       IMAP-FLAGS  = ":flags" string-list
       SPECIAL-USE = ":specialuse" string

    4.  Format of FCC Messages

       Copies of messages filed into a mailbox via this extension are
       REQUIRED to be in the Internet Message Format [RFC5322].  Some
       messages generated by Sieve actions might already conform to this
       format and MAY be filed without modification.  Messages generated in
       other formats MUST be encapsulated using constructs from the Internet
       Message Format [RFC5322] and MIME ([RFC2045], [RFC2046], [RFC2047],
       [RFC2231]).

       The general requirements for encapsulating the copies of messages to
       be filed are as follows:

       o  Date: The Date header field is REQUIRED and SHOULD be set to the
          date and time when the message was generated.

       o  From: The From header field is REQUIRED and SHOULD be set to the
          email address of the owner of the Sieve script, unless explicitly
          overridden by rules for encapsulating a particular message type.

             Per Erratum ID 2035 [Err2035],

             "Informative advice: Users often have multiple email addresses,
             and "the email address of the owner of the Sieve script" may
             offer a choice among several.  If the sieve processor
             recognizes an address belonging to the owner of the Sieve
             script in the To or Cc fields of the input message, then it's

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             better to use that address for the From field of the generated
             message, rather than any other addresses the script's owner may
             also have".

       o  To: The To header field is OPTIONAL and MAY be set to the email
          address of the recipient of the generated message, if available.

       o  Subject: The Subject header field is OPTIONAL and MAY be generated
          by setting the subject to the characters "Fcc: " followed by the
          subject of the message being processed by the Sieve interpreter.

       o  In-Reply-To: The In-Reply-To header field is OPTIONAL and MAY be
          set to the Message-ID of the message being processed by the Sieve
          interpreter.

       o  Message Body: The body of the filed message is REQUIRED and is
          composed of one or more MIME parts containing the generated
          message and any related metadata.  The Content-Type header
          field(s) MUST be set to the appropriate MIME types.  If any of the
          MIME parts include 8-bit or binary data, the Content-Transfer-
          Encoding header field(s) MUST be set accordingly.

    5.  Interaction with the Vacation Action

       This document extends the Vacation [RFC5230] action (see also
       "Seconds" parameter [RFC6131] to optionally store a copy of the auto-
       reply messages into a target mailbox.

       Usage:   vacation [FCC]
                         [":days" number | ":seconds" number]
                         [":subject" string]
                         [":from" string]
                         [":addresses" string-list]
                         [":mime"]
                         [":handle" string]
                         <reason: string>

       Example (using fileinto extensions):

       require ["vacation", "fcc", "mailbox", "special-use", "imap4flags"];

       vacation :days 7
                :from "hemingway@example.com" "Gone Fishin'"
                :specialuse "\\Sent" :create
                :fcc "INBOX.Sent" :flags ["\\Seen"];

       Vacation auto-reply messages are compliant with MIME and can be filed
       into the target mailbox without modification.

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    6.  Interaction with the Notify Action

       This document extends the Notify [RFC5435] action to optionally store
       a copy of the notification messages into a target mailbox.

       Usage:   notify [FCC]
                       [":from" string]
                       [":importance" <"1" / "2" / "3">]
                       [":options" string-list]
                       [":message" string]
                       <method: string>

       Example:

       require ["enotify", "fcc"];

       notify :fcc "INBOX.Sent"
              :message "You got mail!"
              "mailto:ken@example.com";

       Messages generated using the "mailto" [RFC5436] notification method
       are compliant with MIME and can be filed into the target mailbox
       without modification.

       Messages generated by other notification methods (e.g., "xmpp"
       [RFC5437]) MUST be encapsulated per Section 4 before being filed.
       The body of the filed message MUST include the ":message" tagged
       argument and MAY include one or more of the ":from", ":importance",
       or ":options" tagged arguments.  The MIME type(s) of the body part(s)
       used to encapsulate the parameters is an implementation decision.

       An implementation MAY only support the FCC extension in conjunction
       with a subset of the notification methods it supports.  An error
       occurs if the FCC extension is used with a notification method that
       doesn't support it.  Notification methods that support the FCC
       extension can be discovered at runtime using the mechanism described
       in Section 6.1.

    6.1.  Notification-Capability "fcc"

       This document defines "fcc", a new notification-capability value for
       use with the notify_method_capability test (see Section 5 of
       [RFC5435].  For the "fcc" notification-capability, the
       notify_method_capability test can match one of the following key-list
       values:

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       yes
          A copy of the notification message sent using the method
          identified by the notification-uri can be filed into a target
          mailbox.

       no
          A copy of the notification message sent using the method
          identified by the notification-uri cannot be filed into a target
          mailbox.

       Note that the "fcc" notify_method_capability test does not require
       the notification-uri argument to specify anything other than a
       scheme.

       Example:

       require ["enotify", "fcc"];

       if notify_method_capability "xmpp:" "fcc" "yes" {
           notify :fcc "INBOX.Sent"
                  :message "You got mail"
                  "xmpp:ken@example.com?message;subject=SIEVE";
       } else {
           notify :fcc "INBOX.Sent"
                  :message "You got mail!"
                  "mailto:ken@example.com";
       }

    7.  Compatibility with the Reject and Extended Reject Actions

       Implementations MUST NOT allow use of the FCC extension with the
       reject and ereject [RFC5429] actions.  Allowing use of the FCC
       extension with these actions would violate the SMTP [RFC5321]
       principle that a message is either delivered or bounced back to the
       sender.  Namely, the saved copy of the rejection message will contain
       the original message.

       It is an error for a script to use the ":fcc" tagged argument with
       either of the reject or ereject actions.

    8.  Compatibility with Other Actions

       The FCC extension is not compatible with any Sieve action that does
       not generate an additional message on behalf of the user.  It is an
       error for a script to use the ":fcc" tagged argument with any such
       action.

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       Future extensions that define actions that generate additional
       messages on behalf of the user need to describe their compatibility
       with the FCC extension and describe how to MIME-encapsulate the
       message, if required.

    9.  Security Considerations

       In addition to the security considerations in [RFC5228], [RFC5230],
       [RFC5435], and [RFC6131], it should be noted that filing copies of
       generated messages may cause the Sieve script owner to exceed the
       allocated storage (quota) on the mail system, thereby preventing
       delivery of future messages destined for the owner.

    10.  Privacy Considerations

       In addition to the privacy considerations in [RFC5228], [RFC5230],
       [RFC5435], and [RFC6131], it should be noted that a copy of a
       generated message filed into a shared or public mailbox (as opposed
       to a private mailbox) could expose private information about the
       Sieve script owner to third parties.  For instance, users that have
       access to the shared/public mailbox might discover that the Sieve
       script owner is on holiday or might discover the owner's physical
       location.

    11.  IANA Considerations

    11.1.  Registration of New Sieve Extension

       IANA has registered the following Sieve extension in the "Sieve
       Extensions" registry at
       <https://www.iana.org/assignments/sieve-extensions>:

          Capability name: fcc

          Description: Adds the ":fcc" parameter to Sieve action commands
          that generate additional messages.

          RFC number: RFC 8580

          Contact address: The Sieve discussion list <sieve@ietf.org>

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    11.2.  Registration of New Notification-Capability Parameter

       IANA has registered the following notification-capability parameter
       in the "Notification-Capability Parameters" registry at
       <https://www.iana.org/assignments/notification-capability-
       parameters>:

          Capability Name: fcc

          Description: Returns whether a copy of the notification message
          sent using the method identified by the notification-uri parameter
          to the notify_method_capability test can be filed into a target
          mailbox.

          Syntax: Can contain one of two values: "yes" or "no".  Values MUST
          be in lowercase.

          Reference: RFC 8580

          Contact: The Sieve discussion list <sieve@ietf.org>

    12.  References

    12.1.  Normative References

       [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part One: Format of Internet Message
                  Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
                  <https://www.rfc-editor.org/info/rfc2045>.

       [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part Two: Media Types", RFC 2046,
                  DOI 10.17487/RFC2046, November 1996,
                  <https://www.rfc-editor.org/info/rfc2046>.

       [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
                  Part Three: Message Header Extensions for Non-ASCII Text",
                  RFC 2047, DOI 10.17487/RFC2047, November 1996,
                  <https://www.rfc-editor.org/info/rfc2047>.

       [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>.

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       [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
                  Word Extensions: Character Sets, Languages, and
                  Continuations", RFC 2231, DOI 10.17487/RFC2231, November
                  1997, <https://www.rfc-editor.org/info/rfc2231>.

       [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>.

       [RFC5230]  Showalter, T. and N. Freed, Ed., "Sieve Email Filtering:
                  Vacation Extension", RFC 5230, DOI 10.17487/RFC5230,
                  January 2008, <https://www.rfc-editor.org/info/rfc5230>.

       [RFC5232]  Melnikov, A., "Sieve Email Filtering: Imap4flags
                  Extension", RFC 5232, DOI 10.17487/RFC5232, January 2008,
                  <https://www.rfc-editor.org/info/rfc5232>.

       [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>.

       [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
                  DOI 10.17487/RFC5322, October 2008,
                  <https://www.rfc-editor.org/info/rfc5322>.

       [RFC5429]  Stone, A., Ed., "Sieve Email Filtering: Reject and
                  Extended Reject Extensions", RFC 5429,
                  DOI 10.17487/RFC5429, March 2009,
                  <https://www.rfc-editor.org/info/rfc5429>.

       [RFC5435]  Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T.
                  Martin, "Sieve Email Filtering: Extension for
                  Notifications", RFC 5435, DOI 10.17487/RFC5435, January
                  2009, <https://www.rfc-editor.org/info/rfc5435>.

       [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>.

       [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>.

       [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>.

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    12.2.  Informative References

       [Err2035]  RFC Errata, Errata ID 2035, RFC 5230,
                  <https://www.rfc-editor.org/errata_search.php?eid=2035>.

       [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                  DOI 10.17487/RFC5321, October 2008,
                  <https://www.rfc-editor.org/info/rfc5321>.

       [RFC5436]  Leiba, B. and M. Haardt, "Sieve Notification Mechanism:
                  mailto", RFC 5436, DOI 10.17487/RFC5436, January 2009,
                  <https://www.rfc-editor.org/info/rfc5436>.

       [RFC5437]  Saint-Andre, P. and A. Melnikov, "Sieve Notification
                  Mechanism: Extensible Messaging and Presence Protocol
                  (XMPP)", RFC 5437, DOI 10.17487/RFC5437, January 2009,
                  <https://www.rfc-editor.org/info/rfc5437>.

       [RFC6131]  George, R. and B. Leiba, "Sieve Vacation Extension:
                  "Seconds" Parameter", RFC 6131, DOI 10.17487/RFC6131, July
                  2011, <https://www.rfc-editor.org/info/rfc6131>.

    Acknowledgments

       The authors would like to thank the following individuals for
       contributing their ideas and supporting this specification: Ned
       Freed, Stan Kalisch, and Alexey Melnikov.

    Authors' Addresses

       Kenneth Murchison
       FastMail US LLC
       1429 Walnut Street, Suite 1201
       Philadelphia, PA  19102
       United States of America

       Email: murch@fastmailteam.com

       Bron Gondwana
       FastMail Pty Ltd
       Level 2, 114 William Street
       Melbourne, VIC  3000
       Australia

       Email: brong@fastmailteam.com

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