SMPT VERP Service Extension
draft-nurpmeso-smtp-verp-00

Internet Engineering Task Force                         S. Nurpmeso, Ed.
Internet-Draft                                             22 March 2025
Intended status: Informational                                          
Expires: 23 September 2025

                      SMPT VERP Service Extension
                      draft-nurpmeso-smtp-verp-00

Abstract

   This specification makes official D.  J.  Bernstein's Variable
   Envelope Return Paths: VERP.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Variable Envelope Return Path service extension . . . . . . .   2
   3.  Additional parameter for MAIL command . . . . . . . . . . . .   2
   4.  Operational behaviour . . . . . . . . . . . . . . . . . . . .   2
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   3
   Appendix A.  Rationale  . . . . . . . . . . . . . . . . . . . . .   4
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   Since at least 1997 many mailing-list managers (and possibly other
   email configurations) make use of D.  J.  Bernstein's Variable
   Envelope Return Paths, or short, VERP.  As he rightfully wrote, on
   1997-02-01: _Every application of RFC 1891's ORCPT and ENVID can be
   handled with VERPs --- easily, reliably, and right now_.  (This is
   DSNs[RFC3461] at the time of this writing.)  This specification
   defines an according SMTP VERP Service Service Extension.  With it,
   undeliverable mail will reveal the recipient address simply through
   the return path address.

2.  Variable Envelope Return Path service extension

   If a SMTP[RFC5321] server announces VERP in the list of EHLO keywords
   (4.1.1.1, Extended HELLO (EHLO)), then VERP is supported.

3.  Additional parameter for MAIL command

   If VERP is supported, the extended MAIL command (SMTP[RFC5321],
   4.1.1.2, MAIL) takes an additional VERP parameter.

4.  Operational behaviour

   When a server supports VERP, and the client requested its usage for a
   particular mail transaction, then it guarantees that each message
   recipient (SMTP[RFC5321], 4.1.1.2, RECIPIENT (RCPT)) will get a copy
   of the message with a variable envelope return path.

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   The VERP is constructed by encapsulating the address of the
   recipient, separated with a plus sign (U+002B, +), and the commercial
   at (U+0040, @) being replaced with an equals sign (U+003D, =), in the
   MAIL sender address, after the senders local-part, before the senders
   domain.

   |  For example, a recipient _abc@def_ will be encapsulated in the
   |  sender address _zyx@wvu_ as _zyx+abc=def@wvu_.

   The construction of the VERP happens when either a MTA is encountered
   along the hops that does not support the VERP service extension, or
   right before final delivery of an email message to a recipient,
   whichever comes first.

   |  _Informative remark:_ This means that a SMTP server which supports
   |  VERP must take into account the need, and therefore be capable, to
   |  splice a single message with potentially many recipients into many
   |  messages with a single recipient and a dedicated VERP.

5.  IANA Considerations

   This document includes no request to IANA.

6.  Security Considerations

   Today, as by IETF means, SMTP trace headers etc need to be traversed,
   or non-standardized, MTA-specific bounce message content has to be
   parsed in order to find out (the) envelope recipient(s).  Furthermore
   more and more SMTP trace headers are seen which completely hide the
   according information.  With VERP as a standardized extension, bounce
   processing can be made a reliable task.

7.  References

7.1.  Normative References

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

7.2.  Informative References

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, DOI 10.17487/RFC3461, January 2003,
              <https://www.rfc-editor.org/info/rfc3461>.

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Appendix A.  Rationale

   This document only specifies VERP for senders.  It could also be
   specified for recipients, to cover more aspects of the referenced
   DSNs[RFC3461].

   This method of creating variable envelope return paths is in active
   use on the internet for over the quarter of a century.  The use of
   the plus sign and the equals sign as delimiters seem to have not been
   the cause of problems in real life.

Appendix B.  Acknowledgements

   Thanks to Wietse Venema of the postfix MTA, for adding VERP support
   in version 1.1 (released 20020117).  The Exim MTA seems to have
   implemented it pre-Y2K.  D.  J.  Bernstein for documenting VERP, and
   implementing it in his qmail MTA, back in 1997.  (This, however, used
   different delimiters: it used hyphen-minus and the equals sign, which
   is a bad choice for other software as list names etc, they often
   regulary contain hyphen-minus.)

Author's Address

   Steffen Nurpmeso (editor)
   Email: steffen@sdaoden.eu

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