John C Klensin writes:
If I correctly understand the intended use of Delivered-to and
the meaning of "added at the time of delivery", I believe that
the examples in Section 4 are in error. The example does not
show a Return-path and I'm having trouble figuring out a case in
which the Delivered-to header field could be added before the
SMTP processor made whatever it would think of a final delivery
and inserted the Return-path.
Return-Path: and Delivered-To: are traditionally added by the MDA, both.
Even though MTA is handing off a message to the MDA for what the MTA thinks
is the final disposition, the MDA has the final word on that. This is
because MDA-managed forwarding comes into play, think about classic
procmail's forwarding rules.
Return-Path: predates Delivered-To: and I have a dim recollection of stuff
that expected to see Return-Path: as the first line in a message in
mailboxes.
So, when Delivered-To: entered the picture: to preserve this dependency the
MDA adds Delivered-To: after Return-Path:.
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