That's precisely the reason that the MAIL FROM address is exposed in the
message header as return-path - so the address can be used for automatic
responses that are generated after the message leaves the transport/delivery
system.
Ah, this is new in RFC 2821.
Actually, a slightly weaker form of this requirement is present in RFC 1123.
Section 5.2.8 says, in part:
When the receiver-SMTP makes "final delivery" of a message,
then it MUST pass the MAIL FROM: address from the SMTP envelope
with the message, for use if an error notification message must
be sent later ...
and then:
IMPLEMENTATION:
The MAIL FROM: information may be passed as a parameter or
in a Return-Path: line inserted at the beginning of the
message.
RFC 2821 makes it explicit that this has to be done with a return-path field;
an improvement IMO.
Nice, however it doesn't seem to be implemented widely.
Hmm. My impression has been the exact opposite. Specifically, I know that PMDF
and iMS (MTAs I've worked on) add such a field, and I believe sendmail, qmail,
MMDF, PP, exim, and even Exchange do so as well.
Ned