Security :: Spam Filter Software For ISP Environment.

We operate a small ISP and are currently using a debian distro as our gateway server.Recently we have had an increased number of spam issues with customers (not them sending it directly, rather the customer getting infected with a virus/malware and then their computer becoming a bot).I'd like to set up another gateway of sorts to sit after our authentication gateway but before our backbone to provide spam filtering (and hopefully virus filtering) for any traffic passing through which might be email.I've tried searching for any linux based software which would suit, but I'm coming up empty.

Surely there's something already out there which can perform this task.Finally, just to clarify, I'm not talking about spam filtering for email accounts we host ourselves (this is built into our mail server); I'm talking about spam originating from customers PC's which is passing through our gateway (but not our mail server).