The Realtime Blackhole List: mail-abuse.org/rbl. This is a list of known sources of spam. You must subscribe to this service to use it—it's still free, but subscription is necessary for what appears to be mainly legal reasons. This is too valuable a resource to avoid for that reason, so read the pages and decide for yourself.
“Securing Sendmail”: www.sendmail.net/000705securitygeneral.shtml. Describes many built-in security features in sendmail and offers security tips applicable to most sendmail installations.
“Securing Sendmail on Four Types of Systems” by Mark Durham: www.sendmail.net/000710securitytaxonomy.shtml.
Sendmail, 2nd Edition by Bryan Costales with Eric Allman. Sebastopol, CA: O'Reilly & Associates, 1997. The definitive guide to sendmail. Chapters 19 and 34 are of particular interest, as they concern use of the m4 macros (most of the rest of this weighty tome covers the ugly insides of sendmail.cf).
“Setting up sendmail on a Firewall, Part 3” by Carole Fennelly (Unix Insider, June 1, 1999): www.itworld.com/Net/3314/swol-0699-security. Excellent article on running sendmail 8.9 and later in a chroot environment.
“Using New Antispam Features in Sendmail 8.10”: www.sendmail.net/810usingantispam.shtml.
“Using SMTP AUTH in Sendmail 8.10” by Mark Durham: www.sendmail.net/usingsmtpauth.shtml.