Anti-Spam Configuration Control: Instructions for configuring Sendmail to prevent relaying and reject spam.
Anti-spam information: Information, tools and techniques for fighting spam
Blocking Mailed Spam: This article discusses several alternatives available to sites that want to block mailed spam.
Kai's SpamShield: Sendmail add-on that provides rate control and volume limits for Unix-based mail servers. Can stop spam from ISP's own customers, and mail bombers.
milter-regex: milter-regex is a sendmail milter plugin that allows to reject or discard mail based on regular expressions matching SMTP envelope parameters and mail headers and body.
milter.org: A blog focused on sendmail milter API.
The Antispam Home Page: Instructions to automatically filter your e-mail, defend your site from exploitation by the spammers
The Spam Series - Email: A series telling everything about the Internet nuisance, spam, from your About.com Guide.
Using check_* in sendmail 8.8: sendmail 8.8: using the check_* rules to prohibit unauthorized relaying through your sendmail server and to reject UBE from well known addresses