[Dovecot] sieve-extprograms strips Return-Path headers

[Ben Morrow]

The sieve-extprograms extension strips Return-Path headers before
passing mail off to the external program. The patch to remove this
behavious is trivial (and attached), but since it appears to be
deliberate I felt I should investigate further.

I believe this is basically a cut-and-paste error. The code to strip
Return-Path has been there since the sieve-pipe plugin for Dovecot 1.x,
and I believe it was copied from the equivalent code in the 'redirect'
action for passing mail off to sendmail(8). In that case, stripping
Return-Path is correct, since the message is being resubmitted for
delivery (by SMTP or otherwise), and the last step of the new delivery
will (should) add a new Return-Path header with the new envelope
information. In the case of sieve-extprograms, though, the filtering is
happening post-delivery, so a stripped Return-Path will never be
replaced.

Ben

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