The largely grown spam mail in past several years has become a serious problem for all the Internet users. Up-to-date, most countermeasures against spam mails are filtering the incoming mails based on either their IP's or the mail addresses, such as black/white list, or their contents. Such passive blocking cannot reduce the overwhelming bandwidth, however. In this paper, we present a CAPTCHA-based spam mail model that can actively block the bots from sending the e-mail such that reducing both the volume of spam mail and the overcrowding bandwidth can be achieved. The kernel of the proposed model is a CAPTCHA Mail Server that is responsible for automatically telling bots apart from humans, besides supporting POP3 and SMTP. The others are: Client Mail Tool for presenting the test and collecting the answer to/from the mail sender, CAPTCHA protocol for communicating the CAPTCHA test between the Server and the Client Mail Tool, and the Certification Authority for certifying the mail servers.
