libmilter

Hmm. Milter or libmilter is a cool API provided by Sendmail to play around with the content of mails (which was not possible in sendmail up till now) and look for interesting patterns. Depending upon the pattern, one can accept, reject, discard or alter the mail. I know of two interfaces to Milter, PerlMx (in perl) and the Python module for Milter. Another good one is MIMEDefang .