I used to configure and run an e-mail gateway for a large company, then taught on it, and now--sometimes--do e-mail configurations as part of my consulting business. I am the system administrator and postmaster for avolio.com. I teach a course on anti-spam techniques and have tested e-mail firewalls for Information Security Magazine. All to say, I have many reasons to be interested in anti-spam techniques. I recently wrote about how I fight spam. But this week I talked to a company that deserves a mention.
Secluda has an interesting way of dealing with spam with their "InboxMaster." They do not even attempt to guess if something is spam or ham. They just ask the question, "Have you ever sent e-mail to this address before, or have you received and accepted e-mail from the i address?" After a "learning period," every e-mail is checked against this question. The user and administrator had many options, but I will briefly describe the way I would use it. All e-mail from addresses that have written to me before or to whom I have written, gets delivered. A few times a day I would get an email with a list of the messages that have been held pending action. I scan the list looking for legitimate e-mail and tag them to be sent (and, perhaps, tag the address as "trusted"). I can also tag the address as one to always reject.
I think it is worth a look. So much so, that I am going to get a test copy to try out on my e-mail gateway and review. I"ll let you know.