maybe i miss the point of your question, but:
the return-path is not an original mail header.
the receiving MTA writes the SMTP envelope sender (the email address that
is submitted by the sending SMTP client in the "MAIL FROM:" statement into
a line beggining with Return-Path:
you can use the sendmail command to set the envelope sender for an
outgoing message:
sendmail -f sender(_at_)adre(_dot_)ss recipient(_at_)adre(_dot_)ss
and you can also do that in a procmail recipe.
i would think that using sendmail -f excludes using sendmail -t tho,
so you would have to extract the recipient address with formail (if it is
supposed to be the same as in the original mail).
hope this helps
wolfgang
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