Postforward is a mail forwarding utility which aims to complement
the Postfix Sender Rewriting Scheme daemon (PostSRSd).

The downside of using PostSRSd from postfix config is that all mail
is naively rewritten, even when no forwarding is actually performed.
Such rewritten Return-Path addresses may confuse sieve scripts and
other mail filtering software.

This is where Postforward comes in. Instead of rewriting all incoming
mail regardless of final destination, mail systems may be configured
to pipe mail into Postforward only when forwarding needs to happen,
e.g. in a users' .forward when the forwarded-to mailserver requires
it, leaving non-forwarded mail unaltered by PostSRSd.
Postforward will rewrite envelope addresses for piped mail using
PostSRSd itself and re-inject these messages back into the queue,
destined for the forwarding recipient(s).