The effectiveness of current anti-spam systems is limited by the ability of spammers to adapt to new filtering techniques and the lack of incentive for mail domains to filter outgoing spam. This paper describes a new approach to spam protection based on distributed trust management. This is motivated by the fact that the SMTP mail infrastructure is managed in a distributed way by a community of mail domain administrators. A trust overlay architecture and a new protocol are presented. The TOPAS protocol specifies how experiences and recommendations are communicated between a spam filter at each mail domain and its associated trust manager, and between trust managers of different mail servers. A technique for improving mail filtering using these trust measures is also described. Initial simulations indicate the potential of this approach to improve rates of false positives and false negatives in anti-spam systems.
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