Today, over 70% of emails are being sent using SendMail protocol which is by SendMail Inc. I just saw the video at scobleshow.com by robert scoble (i love your voice robert!!), where robert is interviewing the founder of SendMail Inc. and lot of things being discussed in the interview, I liked Spam as a topic to think whenever I get some time. Many things can be applied to filter spams out, though every technique is going to increase the complexity..
Download the video from,
http://media.podtech.net/media/2006/12/PID_001561/Podtech_sendmail.mp4
Around 120MB in size.