Q. How do spam filters work? Even though my Internet provider claims to use them, a lot of junk gets through. Is there anything else I can do to block the spam?

A. Spam filters are lists of rules and conditions under which an incoming e-mail message is screened. In addition to scanning for certain words or phrases, spam filters can be written to look for and reject messages that have invalid return addresses, fake domain names or forged headers, or messages from domains or users known to be junk mailers.

Filters can also reject messages sent in significant bulk or through an open-relay system. (An open relay is a mail server that allows anyone, not just its registered users, to relay messages through it, and spammers love them.)

If your e-mail program allows you to write your own filters or mail rules, you can set up your own custom spam screeners, but this can be time-consuming. There are also plenty of third-party programs and services that offer to help keep the spam out of your mailbox.

Most programs require that you have an e-mail account that uses the POP3 or IMAP protocol, and some programs will not work with Web-based mail. (America Online has a proprietary system that usually won't work with third-party filtering programs but offers its own rudimentary set of tools (keyword: Mail Controls).

Among the spam-blocking programs for Windows is SpamKiller, newly acquired by the security applications provider McAfee.com. The program, which can filter several different POP3 e-mail accounts, costs $29.95. But a free 30-day trial version can be downloaded at www.spamkiller.com.

Basic freeware versions of Spam-Weasel (www.mailgate.com/products/spamweas/sw_feat.asp) and SpamEater (www.spameater.net/spameater.asp) are available for home users with Windows systems. Macintosh users running Mac OS 8 or 9 with the Eudora e-mail program can try Email Magician (available for $35 at www.yav.com /emailmagician.html). Mac OS X devotees can find a list of mail programs and utilities for their systems at www.apple.com/downloads/macosx/internet_utilities.