Tumgreyspf, an external policy checker for the postfix mail
server. It can optionally greylist and/or use spfquery to 
check SPF records to determine if email should be accepted 
by your server.

Because of it's design, legitimate e-mail is never trapped 
or rejected. Only spam and viruses are caught. Since adding 
it to our mail server (which also uses Spam Assassin, ClamAV, 
and an outsourced anti-spam system), our spam level has 
dropped by an order of magnitude.

It uses the file-system as it's database, no additional 
database is required to use it. 

WWW: http://www.tummy.com/Community/software/tumgreyspf/

Marcus Alves Grando <mnag@FreeBSD.org>