ProxSMTP is a flexible tool that allows you to reject, change or log
email based on arbitrary critera.  It accepts SMTP connections and
forwards the SMTP commands and responses to another SMTP server.  The
'DATA' email body is intercepted and filtered before forwarding.
ProxSMTP can be used as a transparent proxy to filter an entire
network's SMTP traffic at the router.

Homepage:
http://thewalter.net/stef/software/proxsmtp/
