This software is still in beta test.
Use at your own risk.
Sendmail includes a facility for plugging in custom mail filters, called milters. It's documented here. Bogomilter uses this facility to call bogofilter on every piece of mail coming into the system.
Are you using bogomilter? There's a mailing list: email@example.com (firstname.lastname@example.org to subscribe). And an announcements-only mailing list: email@example.com (firstname.lastname@example.org to subscribe). Anything send to the announcements list also goes to the regular list, so you don't need to be on both.
This is very abbreviated, intended mainly as a reminder for those who have worked with milters before. If it's your first milter, you should look on the web for more thorough documentation. Also, these instructions are pretty specific to FreeBSD, and will have to be adapted for other OSs.
If you don't see MILTER in the compilation options, you will have to re-build sendmail.
sendmail -d0.1 -bt < /dev/null | grep MILTER
Rebuild and install sendmail.cf.
# bogomilter unix:/var/run/bogomilter.sock
bogomilter_enable="YES" bogomilter_flags="" bogomilter_socket="unix:/var/run/bogomilter.sock"
Is Bogofilter Scaleable? by Greg Louis.
Scalable Centralized Bayesian Spam Mitigation with Bogofilter by Jeremy Blosser and David Josephsen.
ACME Labs / Software / bogomilter