sendmail 8.8 configs are setup so that if procmail is the local delivery
agent, and you have all email for a domain going to one local mailbox, then
the -a switch will be passed the envelope address. this can be gotten at
as $1 within the .procmailrc. i've also seen sendmail code somewhere that
will add a X-envelope-to header to all locally delivered mail so that you
can look at that via procmail (or other processors). unfortunately, i
can't remember where i saw that code. i thought it was the procmail faq,
but i can't find anything in there.