Eigenpoll : Spam fighting software : Abacaís ReceiverNet 

http://www.abaca.com/
The concept of receiver reputation is based on the fact that different people receive different amounts of spam and legitimate email. When analyzing a message, each receiverís percentage of spam versus legitimate email (his or her reputation) is an estimate of whether the message is spam or legitimate. Essentially, if the message is sent to users who typically receive a high percentage of spam, the message is more likely to be spam. However, if the message is sent to users who typically receive a low percentage of spam, the message is more likely to be legitimate. Aggregating the reputations of all recipients of a particular message, therefore, is equivalent to combining those usersí rating power to estimate the legitimacy of the sender and the message. In a receiver reputation system, the key determinant of whether a message is spam or legitimate is not the identity of the sender or the content of the email, but the reputations of the email recipients, individually and collectively.