





1. Abstract

   This document describes an extension to the SMTP service [1], called
   Variable Envelope Return Path (VERP).  The VERP extension implements
   a way of automatically identifying undeliverable mail recipients,
   even when non-delivery reports originate from mail systems that do
   not implement delivery status notifications, as specified in [2] and
   [3].

2. Introduction

   All E-mail software can expect to deal with undeliverable mail.  [2]
   and [3] specify a machine-readable format for delivery status
   notifications (DSNs, or non-delivery reports).  DSNs allow
   undeliverable mail to be handled in a totally automatic fashion,
   without requiring manual intervention.  For example, mailing list
   managers can automatically identify addresses that are no longer
   deliverable, and remove them from the mailing list.

   Although [2] and [3] are now widely implemented, there are still many
   systems that do not use them.  This makes it impractical to
   completely rely on DSNs for automatic mailing list management.
   Undeliverable addresses accumulate quickly even from a very small
   percentage of non-DSN systems.  This results in a non-trivial amount
   of manual work to identify undeliverable addresses and purge them
   from the mailing list.

   Mailing list software began to use VERPs (the acronym stands for
   Variable Envelope Return Path) after DSNs were found to be
   impractical for totally automatic mailing list management.  VERPs are
   an alternative way to handle non-delivery notices.  The advantage of
   VERPs is that they can be made to work automatically, even when non-
   delivery notices are not in the format specified by [2].

   Unfortunately, VERPs require much more bandwidth and network
   resources than DSNs because VERPs cannot be used to send one copy of
   a mailing list message addressed to all the recipients in the same E-
   mail domain.

   This SMTP service extension allows E-mail software to send a single
   VERP message to all addresses in the same mail domain, for as long as
   mail servers, which relay the message, support the VERP SMTP
   extension.

   The VERP message may be eventually relayed to a mail server that does
   not support this extension.  Separate messages - with variable
   envelope return paths - will be sent when this happens.




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   So the worst case scenario results in the same situation where
   traditional VERPs are used right from the start.  The best case
   scenario results in significant savings of network resources and
   bandwidth, from eliminating hundreds (or more) copies of the same
   message.

   Essentially, the VERP extension postpones the generation of multiple
   messages with different return paths as much as possible, until it is
   absolutely required.

2.1 VERP overview

   The traditional VERP message encodes the recipient address as a
   portion of the return address.  When undeliverable mail comes back,
   the mail software decodes the return address (now the recipient
   address) and obtains the address responsible for the non-delivery
   notice.

   For example: mail sent by a mailing list manager to the address
   <john@example.org> carries a return address of <mlist-return-
   john=example.org@domain.com>.  The mailing list software at
   domain.com handles all mail with the local portion of the address
   starting with "mlist-return-".  If a non-delivery notice is generated
   because the address is not deliverable, the mailing list software
   takes the address where the non-delivery report was sent, retrieves
   the remaining portion of the local address, "john=example.org", and
   determines that the undeliverable address was <john@example.org>.

   This does not rely on RFC 1894, and will work for all non-delivery
   notices.

3. Framework for the VERP SMTP transport extension

   This SMTP transport extension [1] is laid out as follows.

      (1) The name of the SMTP transport extension defined here is
          Variable Envelope Return Path.

      (2) The EHLO keyword associated with this extension is VERP.

      (3) The VERP EHLO keyword takes no parameters.

      (4) One optional ESMTP keyword VERP is associated with the MAIL
          FROM command. This parameter takes no values.

      (5) No additional ESMTP verbs are defined by this extension.

      (6) The next section specifies how support for this extension



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          affects the behavior of a server and client SMTP.

4. The VERP SMTP extension

   When a VERP keyword is present in the MAIL FROM command, [4], some
   additional restrictions are imposed on the RFC 822 address [5],
   specified by that MAIL FROM command, and on all RFC 822 addresses in
   the subsequent RCPT TO commands that refer to the same message (that
   is, until the next DATA, RSET, or QUIT command).  The term "VERP
   message" refers to any E-mail message whose MAIL FROM command
   includes the VERP keyword.  The term "VERP-compliant server" refers
   to any E-mail server that supports the Variable Envelope Return Path
   SMTP extension.  When a VERP keyword is present in the MAIL FROM
   command:

       (1) The address specified by the MAIL FROM verb MUST contain at
           least one @ character.

       (2) The address in every RCPT TO verb referring to the same
           message MUST contain at least one @ character.

       (3) The domain portion of the address in the MAIL FROM and RCPT
           TO verbs MUST be compliant with the definition of <domain> in
           [6].  That is, it MUST contain only letters, digits, hyphens,
           and periods.  The domain portion of the address is the one
           that follows the last @ character.

4.1 Delivery failures

   When a VERP-compliant server is unable to deliver a VERP message to
   one or more recipients, the VERP server MUST do one of the following:

        1) Return an RFC 1891 delivery status notification to the return
           address, or:

        2) Transmit a separate non-delivery notice for each failed
           recipient.  The return address for each non-delivery notice
           MUST be the address that’s formed by applying the procedure
           described in section 7 of this document to the return address
           of the message and the failed recipient’s address.  If more
           than one recipient was undeliverable a separate notice MUST
           be sent for each undeliverable address.

5. Final delivery

   Section 4.3.1 of [5] specifies that the mail server performing final
   delivery of a message will generate a Return-Path: header containing
   the return address of the message.



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   This return address MUST be formed by applying the procedure
   described in section 7 of this document to the return address and the
   recipient’s address.

   This also applies if the mail server invokes some other external
   process to handle final delivery, instead of placing the message into
   the recipient’s mailbox. In all cases, the return address specified
   by the mail server to any external environment or process MUST be
   derived by applying the procedure in section 7 to the return address
   and the recipient’s address.

6. Relaying

   When a VERP-compliant server determines that a recipient of a VERP
   message is not a local mailbox, and the message must be relayed to
   another server, the VERP-compliant server MUST:

       (1) If the VERP-compliant server’s local policies require the
           return and/or recipient addresses are to be rewritten, the
           VERP-compliant server MUST make sure that delivery notices
           MUST NOT be sent to a rewritten return address, or reflect a
           rewritten recipient address, unless separate arrangements are
           made with the sender of the VERP message.  This is because
           the sender expects to be able to resolve non-delivery notices
           to some recipient address it knows about (irrespective
           whether the addresses are resolved from a VERP, or from a
           delivery status notification in the format specified by [2]).
           When local policies of the relaying server require the
           recipient or the sender address to be rewritten, the sender
           will obviously be not aware of this rewriting or expansion.
           If a subsequent delivery failure results in a non-delivery
           notice being sent to a VERP containing the rewritten address,
           the sender will not be able to associate the address encoded
           in the VERP with any recipient address it has on file.

           A suggested way to handle this situation is to set specify a
           delivery status notification of "NEVER" - as specified by [3]
           - for any recipient address generated by local aliasing or
           rewriting, and treat the original address as being either
           delivered or relayed, resulting in a "delivered" or "relayed"
           delivery status notification if the original recipient
           address specified a "SUCCESS" notification.

           Note that this clause does not prohibit the relaying server
           to have an established agreement with the sender to act as a
           mailing list exploder, as long as this agreement is handled
           in a transparent way.  For example, the relaying server can
           set its own return address on all VERP messages it exploded,



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           and handle non-delivery notices all by itself.

       (2) If the VERP-compliant server determines that the remote
           server is also a VERP compliant server, the VERP keyword MUST
           be included in the MAIL FROM command used to relay the VERP
           message to the remote server.

       (3) If the remote server is not a VERP compliant server, The VERP
           compliant server SHOULD send a separate copy of the message
           for every recipient.  The return address of each copy of the
           message MUST be formed by applying the procedure described in
           section 7 of this document to the original return address,
           and the address of each individual recipient.  Although the
           message SHOULD NOT be returned as undeliverable, if it is
           then the rules defined in section 4.1 MUST be applied.

           These rules also apply if the SMTP-compliant server
           determines that the VERP message must be forwarded via some
           other protocol to a non-SMTP gateway, unless the non-SMTP
           protocol has equivalent features that are completely
           identical in function to Variable Envelope Return Path SMTP
           service extension (including any translations of E-mail
           addresses to and from the non-RFC822 format).

7. Variable envelope return path encoding

   This encoding method starts with a return address and one recipient
   address.  As mentioned previously, both addresses MUST be valid
   RFC822 addresses, [5], and MUST contain at least one @ character.
   The portion of each address following the last @ character MUST be
   compliant with [6], or must be an explict network address literal,
   for example: john43@[192.68.0.4].

   Let "sdomain" represent the portion of the return address that
   follows the last @ character.

   Let "slocal" represent the portion of the return address that
   precedes the last @ character.

   Let "rdomain" represent the portion of the recipient address that
   follows the last @ character.

   Let "rlocal" represent the portion of the recipient address that
   precedes the last @ character.

   To encode the recipient address within the envelope sender address,
   create an address of the following form:




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          slocal-encodedrlocal=encodedrdomain@sdomain

   Where "encodedrlocal" and "encodedrdomain" are formed by taking
   rlocal and rdomain, respectively, and encoding each one as follows:

        1) Each @, :, %, !, -, [, ], and + character in rlocal is
           replaced by a single ’+’ character followed by two uppercase
           hexadecimal characters whose value is the ASCII code of the
           replaced character.

           Taking into account poorly-written mail relays that ignore
           case-sensitivity, both uppercase and lowercase hexadecimal
           characters SHOULD be recognized when decoding envelope return
           paths.

        2) All other characters are unchanged.  Other characters MAY,
           but SHOULD NOT be also encoded in the same fashion.

   This can be represented using BNF as follows:

     encodedverp: slocal "-" encodedrlocal "=" encodedrdomain "@" sdomain

     encodedrlocal: * (char-literal / char-encoded )

     encodedrdomain: * (char-literal / char-encoded )

     char-literal: any character valid in an RFC821 address [4],
                   except @, :, %, !, [, ], -, and +

     char-encoded: "+" hexdigit hexdigit

     hexdigit: ("0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" /
                "9" / "A" / "B" / "C" / "D" / "E" / "F" /
                      "a" / "b" / "c" / "d" / "e" / "f")

8. Variable envelope return path decoding

   Non-delivery notices for VERP messages will be sent to either the
   original address, <slocal@sdomain>, or to the VERP-encoded address,
   <slocal-encodedrlocal=encodedrdomain@sdomain>.

   Messages sent to <slocal@sdomain> will be RFC 1891-compliant delivery
   status notifications.  These messages will be machine-readable, and
   the mail software will be able to identify failed addresses from the
   RFC 1891 delivery report.

   Non-delivery notices will also be sent to the VERP-encoded address,
   and the mail software will be able to reconstruct the failed address



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   from the VERP-encoded address by simply reversing the steps used in
   encoding:

        1) Extract encodedrlocal and rdomain from the recipient address.
           There will be at least one = character in the encoded portion
           of the return address.  encodedrlocal is everything up to the
           last = character.  Everything following the last = character
           is encodedrdomain.

        2) Replacing all occurrences of "+" followed by two hexadecimal
           digits in encodedrlocal and encodedrdomain with the
           equivalent ASCII character.

        3) Using the decoded rlocal, @, then rdomain.

9. Examples

   Suppose that a VERP-compliant server named "example.com" receives a
   message via the following SMTP conversation (for brevity, non-
   relevant headers have been omitted):

       220 example.com ESMTP
       EHLO domain.com
       250-example.com ESMTP
       250-SIZE
       250-DSN
       250-VERP
       250 HELP
       MAIL FROM:<itny-out@domain.com> VERP SIZE=100
       250 Ok
       RCPT TO:<alex@example.com>
       250 Ok
       RCPT TO:<node42!ann@old.example.com>
       250 Ok
       RCPT TO:<tom@old.example.com>
       250 Ok
       RCPT TO:<lisa@new.example.com>
       250 Ok
       RCPT TO:<dave+priority@new.example.com>
       250 Ok
       DATA
       354 Ok
       From: "John" <john@domain.com>
       Date: Thu, 16 Jan 1997 14:49:31 -0500 (EST)
       Subject: Meeting canceled.

       Today’s 2pm meeting has been rescheduled for tomorrow, 9am, due
       to a scheduling conflict.



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       .

   The message is delivered to the local mailbox for <alex@example.com>.
   The message looks like this:

       Return-Path: <itny-out-alex=example.com@domain.com>
       From: "John" <john@domain.com>
       Date: Thu, 16 Jan 1997 14:49:31 -0500 (EST)
       Subject: Meeting canceled.

       Today’s 2pm meeting has been rescheduled for tomorrow, 9am, due
       to a scheduling conflict.

   The VERP-compliant server at example.com connects to the mail server
   for old.example.com.  old.example.com does not support the Variable
   Envelope Return Path extension.  Therefore, old.example.com receives
   two messages.  The SMTP conversation for the first message is as
   follows:

       220 old.example.com ESMTP
       EHLO example.com
       250-old.example.com ESMTP
       250-SIZE
       250-DSN
       250 HELP
       MAIL FROM:<itny-out-node42+21ann=old.example.com@domain.com>
       250 Ok
       RCPT TO:<node42!ann@old.example.com>
       250 Ok
       DATA
       354 Ok
       From: "John" <john@domain.com>
       Date: Thu, 16 Jan 1997 14:49:31 -0500 (EST)
       Subject: Meeting canceled.

       Today’s 2pm meeting has been rescheduled for tomorrow, 9am, due
       to a scheduling conflict.
       .

   The SMTP conversation for the second message is as follows:

       MAIL FROM:<itny-out-tom=old.example.com@domain.com>
       250 Ok
       RCPT TO:<tom@old.example.com>
       250 Ok
       DATA
       354 Ok
       From: "John" <john@domain.com>



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       Date: Thu, 16 Jan 1997 14:49:31 -0500 (EST)
       Subject: Meeting canceled.

       Today’s 2pm meeting has been rescheduled for tomorrow, 9am, due
       to a scheduling conflict.
       .

   example.com connects to new.example.com and determines that
   new.example.com runs a modern ESMTP server that supports the VERP
   keyword. The SMTP conversation then goes like this:

       220 new.example.com ESMTP
       EHLO example.com
       250-new.example.com ESMTP
       250-SIZE
       250-DSN
       250-VERP
       250 HELP
       MAIL FROM:<itny-out@domain.com> VERP SIZE=100
       250 Ok
       RCPT TO:<lisa@new.example.com>
       250 Ok
       RCPT TO:<dave+priority@new.example.com>
       250 Ok
       DATA
       354 Ok
       From: "John" <john@domain.com>
       Date: Thu, 16 Jan 1997 14:49:31 -0500 (EST)
       Subject: Meeting canceled.

       Today’s 2pm meeting has been rescheduled for tomorrow, 9am, due
       to a scheduling conflict.
       .

10. Security concerns

   All the usual security considerations applicable to SMTP are also
   applicable to this extension.  Relay of VERP messages to non-VERP
   servers requires a single message with many recipients to be exploded
   into many messages with one recipient.  In all cases, however, there
   will never be any additional overhead beyond the resources that are
   required when VERPs are manually implemented by the mail sender,
   instead of the VERP SMTP extension.

   Mail systems which support the VERP extension SHOULD have adequate
   security measures, including blocks against unauthorized access and
   relaying.




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10.1 Vacation programs, and other autoresponders

   "Vacation" type autoresponders are often used in practice.  A
   vacation autoresponder is a program that automatically replies to
   every message, informing the sender that the recipient is on
   vacation, or is generally unavailable at this time.

   Vacation autoresponders MUST NOT generate autoresponses to mailing
   list messages, but people often forget to do set them up to do so.
   Because autoresponses are sent to the same address that’s used to
   receive non-delivery reports, malfunctioning autoresponders result in
   the recipient being removed from mailing lists.

   Advanced autoresponders send automatic replies in the format
   specified by [2], as a "delayed" notification.  DSN-aware software
   will not remove addresses from mailing lists due to delayed
   notifications.

   Section 5 of this document specifies that the mail server MUST
   replace the original return address with a VERP-modified address when
   delivering the message to a mailbox or an external process.

   Therefore it is possible that RFC 1891 reports may also be sent to a
   VERP-encoded address, as specified by sections 5 and 7 of this
   document.  Mail software SHOULD ignore any RFC 1891 "delayed" or
   "success" reports that sent to a VERP-encoded address.  If it is a
   "failed" report, note that the VERP address will be more reliable
   than the address specified in the report itself.























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11. References

       [1] Klensin, J., Freed, N., Rose, M., Stefferud, E., Crocker, D.
           "SMTP Service Extensions", RFC 1425, United Nations
           University, Innosoft International, Inc., Dover Beach
           Consulting, Inc., Network Management Associates, Inc., The
           Branch Office, February 1993

       [2] Moore, K., and G. Vaudreuil, "An Extensible Message Format
           for Delivery Status Notifications", RFC 1894, University of
           Tennessee, Octel Network Services, January 1996.

       [3] Moore, K. "SMTP Service Extension for Delivery Status
           Notifications", RFC 1891, University of Tennessee, January
           1996.

       [4] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
           USC/Information Sciences Institute, August 1982.

       [5] Crocker, D., "Standard for the Format of ARPA Internet Text
           Messages", STD 11, RFC 822, UDEL, August 1982.

       [6] Mockapetris, P., "Domain Names - Implementation and
           Specification", RFC 1035, ISI, November 1987

12. Author’s address

   Sam Varshavchik
   Double Precision, Inc.
   402 Main Street Suite 100-241
   Metuchen, NJ 08840
   <mrsam@courier-mta.com>



















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