from the OpenBSD port:

  milter-regex is a sendmail milter plugin that allows to reject mail
  based on regular expressions matching SMTP envelope parameters and
  mail headers and body.

  In order to build milter-regex, sendmail needs to be compiled with
  milter support, installing the libmilter library.

This is the default for the sendmail in the base system.  Some of the sendmail
ports omit libmilter by default.

This port is in an experimental, untested state on FreeBSD.  Please tell the
maintainer, trevor@FreeBSD.org, about your results, whether successful or
unsuccessful.

WWW:  http://www.benzedrine.cx/milter-regex.html