SMTP VERP Service Extension
draft-nurpmeso-smtp-verp-03

Network Working Group                                   S. Nurpmeso, Ed.
Internet-Draft                                             3 August 2026
Intended status: Informational                                          
Expires: 4 February 2027

                      SMTP VERP Service Extension
                      draft-nurpmeso-smtp-verp-03

Abstract

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   2
   2.  Variable Envelope Return Path service extension . . . . . . .   2
   3.  Additional parameter for MAIL command . . . . . . . . . . . .   2

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   4.  Operational behavior  . . . . . . . . . . . . . . . . . . . .   2
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   4
   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.

1.1.  Conventions and Terminology

   This document uses the terminology of The Open Group Standard Base
   Specifications, Issue 8, Volume 1, Chapter 1.6, Terminology.

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 behavior

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

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   The VERP is constructed by encapsulating the address of the
   recipient, separated by 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 hypothetic recipient _abc@def_ will be encapsulated
   |  in the hypothetic sender address _zyx@wvu_ as _zyx+abc=def@wvu_.

   |  _Informative remark:_ A sender may include additional variable
   |  constructs in its address, which must be taken into account when
   |  constructing the VERP.  For example, if the VERP delimiter plus
   |  sign is already present, a separating hyphen-minus (U+002D, -) is
   |  appended to the VERP-delimited address part, followed by the
   |  otherwise unchanged VERP.  In this example, a hypothetic recipient
   |  _abc@def_ will be encapsulated in the hypothetic sender address
   |  _zyx+bounces-1234@wvu_ as _zyx+bounces-1234-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.

   If the resulting VERP for a message recipient would exceed a SMTP
   size limit ("local-part" must fit in 64 octets, the "reverse-path"
   limit of RFC 821 and RFC 2821 is 256 octets), the recipient shall be
   rejected with reply code 550, in conjunction with enhanced status
   codes[RFC3463] 5.1.4 shall be used.

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 to the uppermost standard-compliant extend.
   With VERP as a standardized extension, bounce processing can be made
   a reliable task for one, and reveal only envelope content that the
   sender intended to reveal.

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7.  Normative References

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

8.  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>.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, DOI 10.17487/RFC3463, January 2003,
              <https://www.rfc-editor.org/info/rfc3463>.

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 Roy Schulz of Utah University for attention.  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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