We're using this to add an X-Envelope-To: header:
:0 fhw
* ^Received:.* by soma\.net .* for \/<[^>]+(_at_)[^>]+>
| formail -I "X-Envelope-To: $MATCH"
But you could just set whatever you wanted to $MATCH.
Of course, all mail processed through our machine has a header like this:
Received: from Campino.Informatik.RWTH-Aachen.DE
(campino.Informatik.RWTH-Aachen.DE [137.226.116.240]) by soma.net
(8.7.5/8.6.12) with ESMTP id GAA02011 for <rusty(_at_)hodge(_dot_)com>; Fri, 18
Apr
1997 06:34:38 -0700 (PDT)
Note the received ... for recepient.
This holds true even on Bcc'd stuff.
--
Rusty Hodge <rusty(_at_)hodge(_dot_)com>, San Francisco