DMTP: Controlling spam through message delivery differentiation

Unsolicited commercial email, commonly known as spam, has become a pressing problem in today's Internet. In this paper we re-examine the architectural foundations of the current email delivery system that are responsible for the proliferation of email spam. We argue that the difficulties in controlling spam stem from the fact that the current email system is fundamentally sender-driven and distinctly lacks receiver control over email delivery. Based on these observations we propose a Differentiated Mail Transfer Protocol (DMTP), which grants receivers greater control over how messages from different classes of senders should be delivered on the Internet. In addition, we also develop a formal mathematical model to study the effectiveness of DMTP in controlling spam. Through numerical experiments we demonstrate that DMTP can effectively reduce the maximum revenue that a spammer can gather; moreover, compared to the current SMTP-based email system, the proposed email system can force spammers to stay online for longer periods of time, which may significantly improve the performance of various real-time blacklists of spammers. Furthermore, DMTP provides an incremental deployment path from the current SMTP-based system in today's Internet.