Incorrect envelope sender with Forward feature

[Samuli Kärkkäinen]

This really belongs to bugzilla, but its HTTP currently times out
answering any requests.

When a message is forwarded using the Forward feature, IMP 2.2.6 sets
the envelope sender to (obviously) the value in the From field of the
message being forwarded. At least pine and mutt set the envelope sender
to the email address of the user forwarding the message.

IMP's behaviour is awkward in the following situation. An IMP user
a@foo.com has sent a message to address b@bar.com, and has gotten a
bounce, refered to as "bounce X". Bounce X has a header line "From:
MAILER-DAEMON@foo.com". User A forwards bounce X somewhere using IMP's
Forward feature, and bounce X bounces; the new bounce is refered to as
bounce Y. Because IMP has set the envelope sender of the forwarded
message to MAILER-DAEMON@foo.com, bounce Y is sent to
MAILER-DAEMON@foo.com instead of the forwarder a@foo.com.

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