The program uses the postfix policy delegation protocol to control access to
the mail system before a message has been accepted (please visit
http://www.postfix.org/SMTPD_POLICY_README.html for more information). It
allows you to choose an action (e.g. reject, dunno) for a combination of
several smtp parameters (like sender and recipient address, size or the
client's TLS fingerprint).

WWW: http://www.postfwd.org/