You're looking for the "envelope to" which is not contained in any way inside the mail message in a guaranteed way, although some MTAs rewrite some of the delivery headers to include it as part of the information.

This is the reason I worked with Joe Pruitt at Teleport to create the "procmail hack" for sendmail (now part of the distro) that permits procmail to access the "envelope to" as $1.

But if you're looking at it already delivered in a mail box, you've lost. You'll have to get to the message further upstream.

-- Randal L. Schwartz, Perl hacker