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Abstract

Spam has become a major problem that is threatening the efficiency of the current email system. Spam is overwhelming the Internet because 1) emails are pushed from senders to receivers without much control from recipients, and 2) the cost for delivering emails is very low. In this paper, we present an anti-spam framework that slows down spammers: by adding delay to email delivery, and by consuming more sender resources. Both delay and resource consumption are controlled based on the likelihood of the source of email messages being a spammer, so that our technique only impact the spammers and has negligible impact on normal email senders. The mechanisms are implemented in the TCP level at the recipient side without requiring any modifications at the sender side. Our evaluations show that selectively delaying connections can effectively slow down a spammer thousands of times when they use a simple setup or use open relays. The mechanism of increasing sender s resource consumption can significantly slow down spammers even when they are spamming from their own optimized servers.