Spammer is an anti-spam mail filter for the popular MTA Sendmail.
It checks all IP-Addresses found in the mail header against a DNS blacklist (such as the one from Easynet,
Spamcop or Osirusoft).
Depending on the configuration, Spammer rejects mails from well-known spam sources or tags them with an additional line in the header and a special prefix in the subject.
Spammer requires Sendmail to be compiled with milter support. If you're using Debian Linux you need to install the libmilter-package.
Please see Spammer's project page for downloads, news etc.