Yesterday, IBM released FairUCE (which stands for "Fair use of Unsolicited Commercial Email") which filters spam by verifying sender identity instead of filtering content. The difference between this and existing technologies in fighting spam is that FairUCE doesnt look at the content of the mail but looks at who sends it and their reputation. It doesnt stop by filtering the email but also tries to attack the spammers by sending the spam back to smmers! Quite agressive!
IBM reports that the technology is not end-user ready but admin's can paly with it. It available for download from alphaWorks!.