Internet Draft                                             Aaron Stone
Document: draft-ietf-sieve-refuse-reject-05           libSieve Project
Intended status: Standards Track                         Matthew Elvey
Expires: April 7, 2008                          The Elvey Partnership,
                                                       Alexey Melnikov
                                                            Isode, Ltd
                                                       October 4, 2007

               Sieve Email Filtering: Reject Extension

Status of this Memo

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   This memo updates the definition of the Sieve mail filtering language
   (RFC draft-ietf-sieve-3028bis-XX.txt) "reject" extension, originally
   defined in RFC 3028.

   A "Joe-job" is a spam run forged to appear as though it came from an
   innocent party, who is then generally flooded by automated bounces,
   Message Disposition Notifications (MDNs), and personal 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 memo updates the definition of the "reject" action to allow
   messages to be refused during the SMTP transaction, and defines the
   "ereject" action to require messages to be refused during the SMTP

   The "ereject" action is intended to replace the "reject" action
   wherever possible.

Table of Contents

  1. Introduction                                                   2
  2. Conventions Used in this Document                              3
  3. Sieve "reject" and "ereject" extensions                        X
     3.1  Action ereject
      3.1.1 Rejecting a message at the SMTP/LMTP protocol level
      3.1.2 Rejecting a message by sending a DSN
     3.2  Action reject
     3.3  "ereject"/"reject" compatibility with other actions
     3.4 How "reject"/"ereject" should generate MDNs
     3.5 How "reject"/"ereject" should perform protocol level refusal
  4. Security Considerations                                        X
  5. IANA Considerations                                            X
     5.1 reject extension registration                              X
     5.2 refuse extension registration                              X
  6. References                                                     X
     6.1 Normative References                                       X
     6.2 Informative References                                     X
  7. Acknowledgments                                                X
  8. Author's Addresses                                             X
  9. Intellectual Property Rights Statement                         X
  10. Full Copyright Statement                                      X
  11. Changes from RFC 3028                                         X
  12. Change Log                                                    X

1.   Introduction

   The Sieve mail filtering language [SIEVE] defined in RFC 3028
   specifies that "reject" action shall discard a message and send a
   Message Disposition Notification [MDN] to the envelope sender along
   with an explanatory message.

   This document updates the definition of the "reject" action to permit
   refusal of the message during the SMTP transaction, if possible, and
   defines a new "ereject" action to require refusal of the message
   during the SMTP transaction.

   Implementations are further encouraged to use spam-detection systems
   to determine the level of risk associated with sending an MDN,
   allowing implementations to silently drop the MDN if the rejected
   message is deemed to be likely spam.

   Further discussion highlighting the risks of generating MDNs and the
   benefits of protocol-level refusal can be found in [Joe-DoS].

2.   Conventions Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 spam or how it should be
   identified, nor to define an email virus or how it should be
   detected.  Implementations are advised to follow best practices
   and keep abreast of current research in these fields.

3.   Sieve "reject" and "ereject" extensions

3.1  Action ereject

      Usage:   ereject <reason: string>

   Sieve implementations that implement the "ereject" action must use
   the "ereject" capability string.

   The "ereject" 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 a message is refused depends
   on the capabilities of the mail component (MDA or MTA) executing the
   Sieve script.  The Sieve interpreter MUST carry out one of the
   following actions (listed in order from most to least preferred),
   SHOULD carry out the most preferable action, and SHOULD fall back to
   lesser actions if a preferred action fails.

     1. Refuse message delivery by sending a 5XX response code
        over SMTP [SMTP] or LMTP [LMTP]. See Section 3.1.1 for more

     2. Discard the message if a return-path verification clearly
        indicates that the message has a forged return-path.

     3. Send a non-delivery report to the envelope sender
        ([REPORT] [DSN]). See Section 3.1.2 for more details.

     The ereject action MUST NOT be available in environments that do
     not support protocol level rejection, e.g. an MUA.

3.1.1 Rejecting a message at the SMTP/LMTP protocol level

   Sieve implementations that are able to reject messages at the
   SMTP/LMTP level MUST do so and SHOULD use the 550 response code. Note
   that if a message is arriving over SMTP and has multiple recipients,
   some of whom have accepted the message, Section 3.1.2 defines how to
   reject such a message.

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

   See Section 3.5 for the detailed instructions about performing
   protocol level rejection.

3.1.2 Rejecting a message by sending a DSN

   An implementation may receive 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 a Sieve "ereject" action or for some other 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.

   Note that according to [DSN], Delivery Status Notifications MUST NOT
   be generated if the MAIL FROM (or Return-Path) is empty.

   The DSN message MUST follow the requirements of [DSN] and [REPORT].
   The action-value field defined in [DSN], Section 2.3.3, MUST contain
   the value "failed".  The human-readable portion of the non-delivery
   report MUST contain the reason string from the "ereject" action and
   SHOULD contain additional text alerting the apparent original sender
   that the message was refused by an email filter.  This part of the
   report might appear as follows:

   Your 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,

3.2  Action reject

   This section updates the definition of the reject action in Section
   4.1 of RFC 3028 and is an optional extension to [SIEVE].

      Usage:   reject <reason: string>

   Sieve implementations that implement the "reject" action must use
   the "reject" capability 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.  Unlike the "ereject" action
   described above, this action would always favor preserving the exact
   text of the refusal reason. Typically the "reject" action refuses
   delivery of a message by sending back an [MDN] to the alleged sender
   (see Section 3.4).  However implementations MAY refuse delivery over
   protocol (as detailed in Section 3.5), if and only if all of the
   following conditions are true:

   1) The reason string consists of only US-ASCII characters
      The reason string contains non-US-ASCII and both client and server
      support and negotiate use of an SMTP/LMTP extension for sending
      UTF-8 responses.
   2) LMTP protocol is used
      SMTP protocol is used and the message contains a single recipient
      or SMTP protocol is used, the message contains multiple recipients
      and all of them refused message delivery (whether using Sieve or

   Script generators SHOULD ensure that a rejection action being
   executed as a result of an anti-spam/anti-virus positive test
   be done using the ereject action, as it is more suitable for such

   Script generators MAY automatically upgrade scripts that previously
   used the reject action for anti-spam/anti-virus related rejections.
   Note that such generators MUST make sure that the target environment
   can support the ereject action.

           require ["reject"];

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

   (Pretend that the reason string above contains some non-ASCII text)

3.3  "ereject"/"reject" compatibility with other actions

   This section applies equally to "reject" and "ereject" actions.
   All references to the "reject" action in this section can be replaced
   with the "ereject" action.

   A "reject" action cancels the implicit keep.

   Implementations MUST prohibit the execution of more than one reject
   in a Sieve script.

   "Reject" MUST be incompatible with the "vacation" [VACATION]
   action. It is NOT RECOMMENDED that implementations permit the use of
   "reject" with actions that cause mail delivery, such as "keep",
   "fileinto", "redirect".
   Making "reject" compatible with actions that cause mail delivery
   violates the RFC 2821 principle that a message is either delivered or
   bounced back to the sender. So bouncing a 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 useful to save copies of rejected messages for later

   Any action that would modify the message body will not have an effect
   on the body of any message refused by "reject" using an SMTP response
   code and MUST NOT have any effect on the content of generated

3.4 Rejecting a message by sending an MDN

   The reject action resends the received message to the envelope sender
   specified by the MAIL FROM (or Return-Path) address, wrapping it in
   a "reject" form, explaining that it was rejected by the recipient.

   Note that according to [MDN], Message Disposition Notifications MUST
   NOT be generated if the MAIL FROM (or Return-Path) is empty.

   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 apparent original sender that mail was refused by an
   email filter.

   The MDN disposition-field as defined in the MDN specification MUST
   be "deleted" and MUST have the "MDN-sent-automatically" and
   "automatic-action" modes set (see Section 3.2.6 of [MDN]).

   In the following script, a message is rejected and returned to the
   alleged sender.

           require ["reject"];

           if header :contains "from" "" {
               reject text:
   I am not taking mail from you, and I don't
   want your birdseed, either!"

   For this script, the first 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,

3.5 How "reject"/"ereject" should perform protocol level refusal

   If the "reason" string consists of multiple CRLF separated lines,
   then 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.

           require ["ereject", "spamtest", "fileinto",

           if spamtest :value "ge"
                       :comparator "i;ascii-numeric" "6" {
               ereject 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 an 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 "ereject" doesn't wish to
   immediately disclose the reason for rejection (for example that it
   detected spam), it may delay immediately sending of the 550 error
   code by sending a 4XX error code on the first attempt to receive
   the message.

4.   Security Considerations

   The Introduction to this document discusses why rejecting messages
   before delivery is better than accepting and bouncing them.

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

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

5.   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 as detailed below:

   Capability name: reject
   Description:     adds the 'reject' action for refusing delivery
                    of a message. The exact reason for refusal is
                    conveyed back to the client.
   RFC number:      this RFC
   Contact address: The Sieve discussion list <>

5.2  ereject extension registration

   IANA is requested to replace the preliminary registration of the
   Sieve refuse extension with the following registration:

   << Issue of replace / obsolete the draft refuse extension:

     Matthew: Would it be better to have it obsolete it, rather
     than replace it? I think so, to prevent inadvertent reuse,
     especially since there are 'refuse' implementations.

     Alexey: I agree with obsoleting it, if you think there are
     implementations. But I thought there were no implementations
     of refuse.


   Capability name: ereject
   Description:     adds the 'ereject' action for refusing delivery
                    of a message. The refusal should happen as early
                    as possible (e.g. at the protocol level) and might
                    not preserve the exact reason for refusal if it
                    contains non-US-ASCII text.
   RFC number:      this RFC
   Contact address: The Sieve discussion list <>

6.   References

6.1  Normative References

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, March 1997.
   << NIT: KEYWORDS is never cited >>

   [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.
   << NIT: LMTP is Informative >>

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

   [MDN] Hansen, T. and G. Vaudreuil, "Message Disposition
   Notification", RFC 3798, May 2004.

   [REPORT] Vaudreuil, G., "The Multipart/Report Content Type for the
   Reporting of Mail System Administrative Messages", RFC 3462,
   January 2003.

   [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,

6.2  Informative References

   [Joe-DoS] Stefan Frei, Ivo Silvestri, Gunter Ollmann, "Mail Non
   Delivery Message DDoS Attacks", 5 April 2004",

   [SPAMTEST] Daboo, C., "SIEVE Email Filtering: Spamtest and
   Virustest Extensions", work in progress, draft-ietf-sieve-

   [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

7.   Acknowledgments

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

8.   Authors' Addresses

   Aaron Stone
   libSieve Project
   260 El Verano Ave
   Palo Alto, CA 94306


   Matthew Elvey
   The Elvey Partnership, LLC
   1819 Polk Street, Suite 133
   San Francisco, CA 94109


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


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11.  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.

  Added the "ereject" action that is similar to "reject", but will
  always favor protocol level message rejection.

12.  Change Log

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

   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
   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 incompatible w/
       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.
   07   (SIEVE WG 04) Removed special handling of empty return path.
       Several editorial changes from Randy Gellens.
       Clarified :exacttext applicability, removed redundancy.  Reverted
       SHOULD NOT send MDNs back to MUST NOT send MDNs of earlier drafts
       (section 3.1.3).
   08   (SIEVE WG 05)
       Reformatted the text to use no more than 72 characters per line.
       Reverted back to two actions (reject and ereject), as per
       consensus at the IETF 67. Major text update/rewrite as the
       result. Changed the order of actions that can be performed by
       ereject: protocol level rejection should always be first,
       followed by "accept and discard" for the case of faked return
       path. Added more details on how DSN reports should be generated.
   09  Editorship of this document taken over by Aaron Stone. Many
       general edits, including clarifications and grammar and spelling
       corrections. Updated boilerplate to RFC 4748. Nits identified.
       Republished for the first time in a long time.