Internet Draft                                                M. Elvey
Document: draft-elvey-refuse-sieve          The Elvey Partnership, LLC
Expires: December 2004                                     A. Melnikov
                                                             Isode Ltd
                                                             June 2004

         The SIEVE mail filtering language - refuse extension
                   draft-elvey-refuse-sieve-02.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Abstract

   This memo defines the SIEVE mail filtering language [SIEVE]
   "refuse" extension.
   A Joe-job is a spam run forged to appear as though it came from an
   innocent party, who is then generally flooded by the bounces, MDNs
   and messages with complaints.  With the Sieve "reject" action, MDNs
   contribute to the flood of Joe-job spam to victims of Joe-jobs;
   SMTP level refusals usually don't.  With "refuse", Sieve gains the
   ability to simply not accept an email during the SMTP transaction
   (instead of accepting it and then sending an MDN [MDN] back to the
   alleged sender using "reject").


Table of Contents

   1. Discussion                                                     2
   2. Conventions Used in this Document                              3
   3. Introduction and Overview                                      3
   4. SIEVE Extension                                                4
      4.1 Action refuse                                              4
      4.2 "refuse" compatibility with other actions                  5
      4.3 Explicit accomodation for servers that support Enhanced
      Error Codes [ENHANCED-CODES]                                   5
   5. Security Considerations                                        5
   6. IANA Considerations                                            6
      6.1 refuse extension registration                              6
   7. References                                                     6
      7.1 Normative References                                       6
      7.2 Informative References                                     7
   8. Acknowledgments                                                7
   9. Author's Addresses                                             7
   10. Intellectual Property Rights Statement                        7
   11. Full Copyright Statement                                      8
   12. Change Log                                                    8

.   Discussion

   The SIEVE mail filtering language [SIEVE] "refuse" extension, if
   supported, permits users to handle unwanted email in a way that is
   sometimes preferable to the existing 'discard' and 'reject'
   capabilities.  When a spam-detection system suspects a message is
   spam, but isn't certain, discarding the email is considered too
   risky for some users, for example, those who receive sales leads by
   email. They are willing to use the reject command. Users are
   willing to reject but not discard because the sender of an email
   incorrectly marked as spam will receive a notification that the
   email was refused, and will likely try again to contact the
   intended recipient, perhaps via another method of communication.
   Unfortunately, this usage is problematic, because in the usual
   case, the email is indeed spam, and the alleged sender to whom the
   MDN caused by the reject will be sent will often be an innocent Joe-
   job victim.  "Refuse" is intended to be superior to "reject"
   because it will be less likely to result in email to an innocent
   victim.  "Refuse" refuses to accept an email for delivery instead
   of accepting it and then sending an MDN.  Much spam is sent through
   open proxies, so "refuse" reduces Joe-job bounces resulting from
   usage of reject. "Refuse" will also reduce Joe-jobs caused by virus
   self-propagation via emails with false sender information.
   "Refuse" may conserve bandwidth, by reducing the number of MDNs
   sent. Further discussion highlighting the risks of "reject" and the
   benefits of "refuse" can be found in [Joe-DoS].


.   Conventions Used in this Document

   Conventions for notations are as in [SIEVE] section 1.1, including
   use of [KEYWORDS].

   This document does not attempt to define what exactly constitutes a
   spam or virus containing email or how it should be identified, or
   what actions should be taken when detected.


.   Introduction and Overview

   The "refuse" action MUST refuse to accept an email for delivery at
   the SMTP/LMTP level by returning a 5XX reply code, instead of
   sending an MDN as required by the "reject" action, other than for
   the two exceptions specified below.  A SIEVE implementation that
   cannot do so MUST NOT claim to support the refuse extension.

   There is an exception when a message has multiple valid recipients,
   and at least one but not all of them are refusing delivery (whether
   the refusal is caused by execution of a Sieve "refuse" or for
   another reason). In this case, the server MUST accept the message
   and generate DSNs for all recipients that are refusing it. Note
   that this exception only applies to SMTP, as LMTP is able to reject
   messages on a per-recipient basis.

   If a "refuse" implementation performs a return-path verification
   and it clearly indicates that the message has a forged return-path,
   the implementation need not refuse to accept the mail, but rather
   MAY accept and discard it.


.   SIEVE Extension

   This section defines the "refuse" action.

4.1  Action refuse

      Syntax: refuse <reason: string>

   The "refuse" action refuses delivery of a message by sending back
   the 550 SMTP response code to an SMTP client.

   This extension can be only supported by a Sieve implementation
   running in a MTA.

   Note that SMTP [SMTP] doesn't allow for non-ASCII characters in
   SMTP response text. It is an error for non-ASCII characters to
   appear in the "reason" string (unless the client and the server use
   an SMTP extension that allows for transmission of non-ASCII reply
   text; such an extension is not known to the authors).

   If the "reason" string is multiline, than the reason text MUST be
   returned as a multiline SMTP/LMTP response, per [SMTP], section
   4.2.1.

   In the following script (which assumes support for the spamtest
   extension), messages that test highly positive for spam are
   refused.

      Example:
      require ["refuse", "spamtest"]

      if spamtest :value "ge" :comparator "i;ascii-numeric" "6" {
                   refuse text:
   SpamAssassin thinks the message is spam.
   It is therefore being refused.
   Please call 1-900-PAY-US if you want to reach us.
   .
                            ;
      elsif spamtest :value "ge" :comparator "i;ascii-numeric" "4" {
                   fileinto "Suspect";
      }

   The following excerpt from an SMTP session shows it in action.
   .
   C: DATA
   S: 354 Send message, ending in CRLF.CRLF.
    ...
   C: .
   S: 550-SpamAssassin thinks the message is spam.
   S: 550-It is therefore being refused.
   S: 550 Please call 1-900-PAY-US if you want to reach us.

   SIEVE implementations that implement the "refuse" action must use
   the "refuse" capability string.


4.2  "refuse" compatibility with other actions

   "Refuse" cancels the implicit keep, and is incompatible with
   "reject" and "discard". "Refuse" is also incompatible with
   "vacation" extension [VACATION]. (It should be compatible and
   incompatible with the same actions as "reject", but [SIEVE] states
   "Implementations SHOULD prohibit reject when used with other
   actions." However we feel that "refuse" should be permitted when
   used with other actions such as "fileinto" and "redirect".  This
   could be useful for analyzing, tracking or reporting spam.  Also,
   users can use tricks (such as multiple redirects back to their own
   email addresses) to get around such a prohibition anyway.)


4.3  Explicit accomodation for servers that support Enhanced Error
    Codes [ENHANCED-CODES]

   This section only concerns implementations that support Enhanced
   Error Codes.

   If the server supports RFC 2034 [ENHANCED-CODES] it MUST select an
   appropriate Enhanced Error Code (e.g. 5.7.1 or a more generic
   5.7.0) and prepend it to the "reason" text. I.e. on such an
   implementation, the example in section 4.1 would show up in SMTP
   as:

     550-5.7.1 SpamAssassin thinks the message is spam.
     550-5.7.1 It is therefore being refused.
     550 5.7.1 Please call 1-900-PAY-US if you want to reach us.

   if the server selected "5.7.1" as appropriate.


.   Security Considerations

   The "refuse" extension does not raise any security considerations
   that are not present in the base [SIEVE] protocol, and these issues
   are discussed in [SIEVE].


.   IANA Considerations

   The following section provides the IANA registration for the Sieve
   extensions specified in this document:


6.1  refuse extension registration

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

   Capability name: refuse
   Capability keyword: refuse
   Capability arguments: N/A
   Standards Track/IESG-approved experimental RFC number: this RFC
   Person and email address to contact for further information:

       Matthew Elvey
       The Elvey Partnership, LLC
       3042 Sacramento-ietf St Ste 04
       San Francisco, CA
       U.S.A.

       <mailto:sieve3@matthew.elvey.com>


.   References

7.1  Normative References

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

   [SIEVE] Showalter, "Sieve: A Mail Filtering Language", RFC 3028,
   January 2001.

   [SMTP] Klensin, J. (Editor), "Simple Mail Transfer Protocol", AT&T
   Laboratories, RFC 2821, April 2001.

   [LMTP] Myers, J., "Local Mail Transfer Protocol", Carnegie-Mellon
   University, RFC 2033, October 1996.

   [DSN] Moore , K., Vaudreuil, G., "An Extensible Message Format for
   Delivery Status Notifications", University of Tennessee, Lucent
   Technologies, RFC 3464, January 2003.

   [MDN] Fajman, R., "An Extensible Message Format for Message
   Disposition Notifications", National Institutes of Health, RFC
   2298, March 1998.

   [ENHANCED-CODES] Freed, N., "SMTP Service Extension for Returning
   Enhanced Error Codes", Innosoft, RFC 2034, October 1996.


7.2  Informative References

   [Joe-DoS] Stefan Frei, Ivo Silvestri, Gunter Ollmann, "Mail Non
   Delivery Message DDoS Attacks", 5 April 2004;
   http://www.techzoom.net/paper-mailbomb.asp


.   Acknowledgments

   Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen and Mark E.
   Mallett for comments and corrections.


.   Author's Addresses

   Matthew Elvey
   The Elvey Partnership, LLC
   3042 Sacramento-ietf St Ste 04
   San Francisco, CA
   U.S.A.

   Email: sieve3@matthew.elvey.com


   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex, TW12 2BX
   UK

   Email: Alexey.Melnikov@isode.com


.  Intellectual Property Rights Statement

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   The IETF takes no position regarding the validity or scope of any
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   to obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification
   can be obtained from the IETF Secretariat.


.  Full Copyright Statement

   Copyright (C) The Internet Society 2004.  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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.  Change Log

   00   First formal draft.
   01   Explicit RFC 2034 support, disallow "refuse" in MUAs, typos
        corrected, clarifications, etc.
   02   Many insubstantial editorial changes (mostly rewording text for
        readability). Added text regarding non-ASCII characters in the
        refuse "reason" string. Added an exception allowing return-path
        forgery to justify discarding a message.