]> SMPT VERP Service Extension
steffen@sdaoden.eu
 General Internet Engineering Task Force SMTP This specification makes official D. J. Bernstein's Variable Envelope Return Paths: VERP.
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 &mydash; easily, reliably, and right now. (This is DSNs 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.
Variable Envelope Return Path service extension If a SMTP server announces VERP in the list of EHLO keywords (4.1.1.1, Extended HELLO (EHLO)), then VERP is supported.
Additional parameter for MAIL command If VERP is supported, the extended MAIL command (SMTP, 4.1.1.2, MAIL) takes an additional VERP parameter.
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, 4.1.1.2, RECIPIENT (RCPT)) will get a copy of the message with a variable envelope return path. 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.
Informative remark: A sender may include additional variable constructs in its address, which must be taken into account when constructing the VERP. For example, a recipient abc@def will be encapsulated in the 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.
IANA Considerations This document includes no request to IANA.
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.
References Normative References Informative References
Rationale This document only specifies VERP for senders. It could also be specified for recipients, to cover more aspects of the referenced DSNs. 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.
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.)