Throttling Outgoing SPAM for Webmail Services

Spam has become a serious problem of Internet, and the current defense is limited to the filters deployed at the recipient side. Little known research has been applied to reduce the volume of spam messages being generated. In this paper, we present a system that dynamically throttles emails based on the message content at the email server provider (ESP) side. The goal of this system is to reduce the spam generated by the ESP while not introducing long delay to legitimate messages. This goal is achieved by applying spam filters during the email delivery time and by using filter scores to control the throttling effect. The throttling effect is implemented through a computational puzzle system. We present experiments and results that show the effectiveness of this anti-spam system that under state of the art hardware, we can limit the ability of the spammer even though he possesses 1000 times as many CPU resources as the normal sender.