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 its 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 its database, no additional database is required to
use it.