John Hentsch wrote in a message to Tony Langdon:

 JH> I'm no expert field but how about putting a special field in the
 JH> header or string at the top of the message containing the actual
 JH> recipient. This could allow a utility program to process the
 JH> message and rename it to proper person. Sort's like the "to:" in a
 JH> gated netmail message which, BTW, was never designed to transport
 JH> email messages but is done every day by hundreds of folks...:)


If you come up with the sendmail rules that it would take 
to do this, I'll add support into it for GIGO after verifying
that it works with our local copy of sendmail here.  After
that point, GIGO would be able to support pop packets *only* when
that line containing the envelope information is available.

I'd suggest the header name be "X-Envelope-To: " .. multiple
addresses wouldn't be a problem.  Alternately, "X-Destination"
or something of that sort.  I'd have GIGO strip it out to
ensure the privacy of "Bcc"'s.
 
cheers,
 jason