FYI, there are Procmail/Smartlist scripts available at:
http://www.johncon.com/john/Cmd/
which is a Certified Mail Delivery agent intended to reduce UCE/spam.
The way it works is that a database "whitelist" is maintained of all the legitimate e-mail addresses that have passed through an e-mail server/gateway, (using the bsearchtext database available at http://www.johncon.com/john/receivedIP/).
If a message's e-mail address is not in the "whitelist", the message is archived, (via Smartlist scripts-like an archiving mailing list,) and a e-mail sent back to the sender requesting a reply. If the sender replies, (i.e., in doing so, certifies the message as authentic,) the Smartlist script forwards the original message on to the recipient, and enters the e-mail address in the "whitelist" database.
Although technically feasible, my tests show that it is annoying in practice, and few senders reply to the certification request.
But it does almost totally block UCE/spam.
YMMD, FWIW ...
John