While searching for an easy way to incorporate SpamAssassin in our organisation we were looking for a proxyserver/firewall kind of way.
If you are looking for something more basic that just does basic filtering (no user specific filtering!) you can consider assasind.
Assasind can easyly be put inbetween your NAT and your propriety mail server, work 'on' your mail server or if you have a separate firewall running Linux you can place assassind there.
Assasind will listen at the appointed port (default:2025) for incoming SMTP traffic. Depending on the settings, mail will be scanned by SpamAssassin and/or Fprot for viruses. When done, assassind will forward it to the real mail server (default localhost:25)
From the manual :
assassind is a relaying SMTP proxy that filters spam using SpamAssassin. The proxy is designed to be robust in the face of exceptional errors, and will (hopefully) never lose a message. In addition to scanning for spam, assassind can optionally scan for viruses using F-Prot.
Thanks to Rude Dog for creating this script.