Internet Draft                                                M. Elvey
Document: draft-ietf-sieve-refuse-reject-03     The Elvey Partnership,
                                                                   LLC
Expires: December 2006                                     A. Melnikov
                                                             Isode Ltd
                                                             June 2006



         The SIEVE mail filtering language - reject extension
                    draft-ietf-sieve-refuse-reject-03.txt

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Abstract

   This memo defines the SIEVE mail filtering language (RFC
   <<3028bis>>) "reject" 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,
   Message Disposition Notifications (MDNs) and messages with
   complaints.  The original Sieve "reject" action defined in RFC 3028
   required use of MDNs for rejecting messages, thus contributing to
   the flood of Joe-job spam to victims of Joe-jobs.  This document
   updates definition of "reject" to require rejecting messages during
   the SMTP transaction (instead of accepting them and then sending
   MDNs back to the alleged sender) wherever possible, thereby
   reducing the problem.


Table of Contents

  1. Introduction                                                   2
  2. Conventions Used in this Document                              3
  3. SIEVE "reject" extension                                       3
     3.1 Action reject                                              3
     3.2 "reject" compatibility with other actions                  7
  4. Security Considerations                                        7
  5. IANA Considerations                                            7
     5.1 reject extension registration                              7
     5.2 refuse extension registration                              8
  6. References                                                     8
     6.1 Normative References                                       8
     6.2 Informative References                                     8
  7. Acknowledgments                                                9
  8. Author's Addresses                                             9
  9. Intellectual Property Rights Statement                         9
  10. Full Copyright Statement                                     10
  11. Changes from RFC 3028                                        11
  12. Change Log                                                   11


.   Introduction

   The SIEVE mail filtering language [SIEVE] "reject" action defined
   in RFC 3028 only allowed users to refuse delivery of a message by
   sending an [MDN].

   This document updates definition of the "reject" action to permit
   users to handle unwanted email in a way that is sometimes
   preferable to the existing 'discard' and the original '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 an
   MDN caused by the reject will be sent will often be an innocent Joe-
   job victim. The updated "reject" is less likely to result in email
   to an innocent victim, because it requires that an implemention
   refuse to accept an email for delivery instead of accepting it and
   then sending an MDN wherever possible.  Much spam is sent through
   open proxies, so SMTP level refusal reduces Joe-job bounces (AKA
   backscatter) resulting from usage of MDNs. The updated "reject"
   will also reduce Joe-jobs caused by virus self-propagation via
   emails with false sender information.  SMTP level refusal helps to
   prevent the blacklisting of sources of backscatter and conserve
   bandwidth, by reducing the number of MDNs sent. Further discussion
   highlighting the risks of generating MDNs and the benefits of
   protocol level refusal can be found in [Joe-DoS].


.   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 RFC 2119.

   Conventions for notations are as in [SIEVE] section 1.1.

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


.   SIEVE "reject" extension

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


3.1  Action reject

      Usage:   reject <reason: string>

   The "reject" action cancels the implicit keep and refuses delivery
   of a message. The reason string is a UTF-8 [UTF-8] string
   specifying the reason for refusal. How message is refused depends
   on capabilities of mail component (MUA, MDA or MTA) executing the
   Sieve script. The Sieve interpreter must do one of the following
   actions, as detailed by the following priority table (items listed
   earlier take precedence). Note that if action can not be taken or
   fails, the interpreter should try the next item in the list:

     1.   If message return-path (MAIL FROM) is empty the message SHOULD
        be accepted and discarded.
     2.   If a "reject" implementation performs a return-path verification
        and it clearly indicates that the message has a forged return-path,
        the implementation need not refuse mail delivery, but rather MAY
        accept and discard it.
     3.   Message delivery is refused by sending 5XX response code over
        SMTP [SMTP] or LMTP [LMTP]. See section 3.1.1 for more details.
     4.   Message delivery is refused by sending a non delivery report
        (DSN [DSN]). See section 3.1.2 for more details.
     5.   Message delivery is refused by sending a message disposition
        notification report (MDN). See section 3.1.3 for more details.


3.1.1 Rejecting messages at SMTP/LMTP protocol level

   Sieve implementations that are able to reject messages at SMTP/LMTP
   level SHOULD use the 550 response code. Note that if a message is
   arriving over SMTP and has multiple recipients, some of which have
   accepted the message, or the Sieve implementation is part of an
   MUA, section 3.1.2 and section 3.1.3 define how to reject such a
   message.

   Note that SMTP [SMTP] doesn't allow for non-ASCII characters in
   SMTP response text. If non-ASCII characters appear in the "reason"
   string, they can be sent if and only if the client and the server
   use an SMTP extension that allows for transmission of non-ASCII
   reply text. (One example of such SMTP extension is described in
   [UTF8-RESP].) In the absence of such SMTP extension, Sieve engine
   MUST replace any reason string containing non-ASCII characters
   with an implementation defined ASCII-only string. Implementations
   SHOULD notify user that such replacement took place.
   Users that don't like this behavior should consider using
   "reject :exacttext" as described in Section 3.2, if available.

   If the "reason" string consist of multiple CRLF separated lines,
   than the reason text MUST be
   returned as a multiline SMTP/LMTP response, per [SMTP], section
   4.2.1. Any line MUST NOT exceed the SMTP limit on the maximal line
   length. To make the reason string conform to any such limits the
   server MAY insert CRLFs and turn the response into a multiline
   response.

   In the following script (which assumes support for the spamtest
   [SPAMTEST] and fileinto extensions), messages that test highly
   positive for spam are refused.

      Example:
      require ["reject", "spamtest",
      "comparator-i;ascii-numeric", "fileinto"];

      if spamtest :value "ge" :comparator "i;ascii-numeric" "6" {
          reject text:
   AntiSpam engine thinks your 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-AntiSpam engine thinks your message is spam.
   S: 550-It is therefore being refused.
   S: 550 Please call 1-900-PAY-US if you want to reach us.

   If the SMTP/LMTP server supports RFC 2034 [ENHANCED-CODES] it MUST
   prepend an appropriate Enhanced Error Code to the "reason" text.
   Enhanced Error code 5.7.1 or a more generic 5.7.0 are RECOMMENDED.
   With Enhanced Error Code the response to DATA command in the SMTP
   example below will look like:

     S: 550-5.7.1 AntiSpam engine thinks your message is spam.
     S: 550-5.7.1 It is therefore being refused.
     S: 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.

   If a Sieve implementation that supports "reject" doesn't wish to
   immediately disclose the reason for rejection (for example that it
   detected spam), it may delay immediate sending the 550 error code
   by sending a 4XX error code on the first attempt to receive the
   message.


3.1.2 Rejecting message by sending DSN

   If the implementation receives a message via SMTP that has more
   than one RCPT TO that has been accepted by the server, and at least
   one but not all of them are refusing delivery (whether the refusal
   is caused by execution of a Sieve "reject" 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
   does not apply to LMTP, as LMTP is able to reject messages on a per-
   recipient basis.


3.1.3 Rejecting message by sending MDN

   When Sieve engine is running inside MUA it has no ability to reject
   the message before it was delivered, as the message is already
   delivered. In this case the client should send a Message
   Disposition Notification [MDN] back to the sender. It resends the
   message to the sender as specified in the Return-Path header field,
   wrapping it in a "reject" form, noting that it was rejected by the
   recipient.
   MTAs and MDAs SHOULD NOT implement "reject" by sending MDNs, they
   SHOULD reject at protocol level as described in section 3.1.1.
   In the following script, a message is rejected and returned to the
   sender.

   Example:
                require ["reject"];

                if header :contains "from" "coyote@desert.example.org"
   {
                reject text:
   I am not taking mail from you, and I don't
   want your birdseed, either!"
   .
                ;
             }

   A reject message MUST take the form of a failure MDN as specified
   by [MDN]. The human-readable portion of the message, the first
   component of the MDN, contains the human readable message
   describing the error, and it SHOULD contain additional text
   alerting the original sender that mail was refused by a filter.
   This part of the MDN might appear as follows:

   ------------------------------------------------------------
   The message was refused by the recipient's mail filtering program.
   The reason
   given was as follows:

   I am not taking mail from you, and I don't want your birdseed,
   either!
   ------------------------------------------------------------

   The MDN action-value field as defined in the MDN specification MUST
   be "deleted" and MUST have the MDN-sent-automatically and automatic-
   action modes set.

3.2  :exacttext optional argument to reject action

   SIEVE implementations that implement the :exacttext optional argument
   to the "reject" action must advertise "rejectexact" capability in
   addition to the "reject" capability described above.

   The :exacttext argument affects how reject processing described in
   section 3.1.1 is performed. When this argument is present, SMTP
   client and server don't support an SMTP extension that allows for
   transmission of non-ASCII reply text and there is non-ASCII text
   in the reason string, then the reason string MUST NOT be replaced
   with an implementation defined ASCII-only string as defined in 3.1.1.
   Instead, Sieve engine MUST try to generate DSN, in order to preserve
   the exact text specified in the reason string.

   Example:
                require ["reject", "rejectexact];

                if size :over 100K {
                reject :exacttext text:
   Your message is to big. If you want to send me a big attachement,
   put it on a public web site and send me an URL.
   .
                ;
             }

   <<Pretend that the reason string above contains some non-ASCII text>>

   NOTE: The :exacttext optional argument doesn't affect reject, if
   Sieve engine is running in MUA, or if Sieve engine is running in
   MTA/MDA, but it also supports an SMTP/LMTP extension for sending
   UTF-8 responses.


3.3  "reject" compatibility with other actions

   A "reject" action cancels the implicit keep.

   Implementations MUST prohibit the execution of more than one reject
   in a SIEVE script. "Reject" is also incompatible with the
   "vacation" [VACATION] extensions.

   Implementations MUST prohibit the execution of more than one reject
   in a SIEVE script. "Reject" MUST be incompatible with the "vacation"
   [VACATION] action. Implementations SHOULD prohibit the use of
   "reject" with actions that cause mail delivery, such as "keep",
   "fileinto", "redirect". <<What about discard?>>
   Making "reject" compatible with actions that cause mail delivery
   violates RFC 2821 principal that a message is either deliver or
   bounced back to the sender. So bouncing message back (rejecting)
   and delivering it will make the sender believe that the message was
   not delivered.
   However there are existing laws requiring certain organizations to
   archive all received messages, even the rejected ones. Also, it can be
   quite convenient to save copies of rejected messages for later
   analysis.

   Any action that would modify the message body will not have effect
   on the body of any message refused by "reject" using the 550 SMTP
   response code and MUST NOT have any effect on context of generated
   DSN/MDNs.


.   Security Considerations

   The Introduction section talks about why rejecting messages before
   delivery is better then accepting and bouncing them.

   Security issues associated with mail auto-responders are fully
   discussed in the security consideration section of [RFC3834]. This
   document is believed not to introduce any additional security
   considerations in this general area.

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


.   IANA Considerations

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

5.1  reject extension registration

   IANA is requested to update the registration for the SIEVE "reject"
   extension to point to this document.
   IANA is also requested to update Tim Showalter's email address to
   be
     tjs@psaux.com


5.2  refuse extension registration

   IANA is requested to remove registration of the refuse extension.
   <<Should this be taken care of by talking directly to IANA?>>


5.3  rejectexact extension registration

   IANA is requested to add the following registration to the list of
   Sieve extensions:

   Capability name: rejectexact
   Description:     adds the ':exacttext' optional argument to the
                    reject action, which instructs Sieve engine to
                    generate Delivery Status Notifications if rejection
                    reason string contains non-ASCII text.
   RFC number:      this RFC (Sieve base spec)
   Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>


.   References

6.1  Normative References

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

   [SIEVE] Showalter, T. and P. Guenther, "Sieve: An Email Filtering
   Language", Work-in-progress, draft-ietf-sieve-3028bis-XX.txt

   [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.

   [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
   RFC 3629, November 2003.

   [VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering:
   Vacation Extension", work in progress, draft-ietf-sieve-vacation-XX.txt.

6.2  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>.

   [SPAMTEST] Daboo, C., "SIEVE Email Filtering: Spamtest and
   Virustest Extensions", work in progress, draft-ietf-sieve-
   spamtestbis-XX.txt
   <<Note to the RFC editor: this reference can be safely replaced
   with RFC 3685.>>

   [RFC3834] Moore, K., "Recommendations for Automatic Responses to
   Electronic Mail", RFC 3834, August 2004.

   [UTF8-RESP] A. Melnikov (Ed.), "SMTP Language Extension",
   work in progress, draft-melnikov-smtp-lang-XX.txt


.   Acknowledgments

   Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen, Kristin Hubner,
   Mark E. Mallett, Philip Guenther and Michael Haardt for comments
   and corrections.

   The authors gratefully acknowledge the extensive work of Tim
   Showalter as the author of the RFC 3028, which originally defined
   the "reject" action.


.   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


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.  Changes from RFC 3028

  Clarified that the "reject" action cancels the implicit keep.
  Extended list of allowable actions on reject to include protocol
  level message rejection and generation of DSNs.


.  Change Log

  <<NOTE to the RFC editor: please delete this section before
  publication.>>

   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.
   03   (Renamed to be SIEVE WG 00) - Updated boilerplate, added reject
       action from the base spec, acknowledged Tim as the author of
       "reject".
   04   (SIEVE WG 01) Based on WGLC feedback, the refuse and the reject
       actions were merged into a single action called reject. Text
       reorganized as the result. Typos and examples corrected. Updated
       IANA registration and Security Considerations sections.
   05   (SIEVE WG 02) Copied some security considerations from Vacation
       draft. Clarified that the "reason" string is in UTF-8. Clarified
       interaction with "editheader" extension. Added text about sending
       of 4XX instead of 550. Corrected typos in several examples.
   06   (SIEVE WG 03) Explicitly list all actions incompable with reject.
       Added two paragraphs explaining why reject SHOULD (as opposed to
       MUST/MAY) be incompatible with them. Clarified that if the reason
       string contains non-ASCII and rejection over protocol is possible,
       then the reason string MUST be replaced with an implementations
       defined ASCII-only string. Added :exacttext optional argument that
       preserves UTF-8 reason string by forcing generation of DSN.