The first step is to block E-mail from known bad addresses. Cassingham has spent considerable time and money boosting his mailing list privacy and security. Tables of blocked addresses, usually called killfiles, are an easy and inexpensive way of dealing with spam. As spammers move to other addresses, of course, you must keep adding to the killfile. Blocks can be set at the firewall based on the Internet Protocol addresses, at the SMTP server, and in most E-mail client applications.
Authentication is the next step up the ladder.
Each incoming E-mail message can be verified by checking the authenticity of the supposed sender. Microsoft's Exchange Server plans to incorporate this feature before the start of next year. But this approach takes time, delays message processing, and can't filter out E-mail bounced off legitimate sites or those with forged return addresses. FireWall-1 from Check Point Software Technologies Ltd. in Redwood Shores, Calif., takes this authentication one step further. If demanded, each incoming message can be held at the proxy connection on the firewall until the sender replies to a query.