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/