🚀 go-pugleaf

There are two classes of reply: error bounces and human-like replies. 
There are different rules for each class.

For human-like replies, follow RFC822 and use Reply-to: if it exists, or
From:.  Do NOT fall back on Sender: or Return-path: or From_.  Remember,
what's happening here is that a human is supposed to really be doing the
addressing, and a user-agent program is simply offering an assist.  If
the user-agent program can't find the proper header to suggest as the
default answer, then it shouldn't be guessing on substandard
information.  The user can do that guessing once the UA program has
announced that the correct headers aren't there.  (Variation: you can
also use the Resent-Reply-to: (falling back on Resent-From:)
information, possibly for a different class of reply.  Still other
classes of replies might also include the To:/CC:/Resent-To:/Resent-CC:
addresses.)

For error bounces, there's an independent collection of hair.  In a pure
RFC822 world, you'd use the Sender: address, falling back on From:
should Sender: not exist.  As implemented in the RFC821 world (using
SMTP), you'd use the ``envelope From'' information, which comes across
as the argument to SMTP's ``MAIL FROM:'' command.  This information is
supposed to be recorded in the Return-path: field when a message is
finally delivered; depending where the rejection happens, it may be
before final delivery occurs.  I believe that the comparable information
is recorded in From_ lines in UUCP-land, but I'm a UUCP-mail novice.

		Craig Everhart