Email anti-spam filtering service

I am looking for an enterprise anti-spam solution that will run/be managed external to the company, yet allow local fine tuning of anti-spam algorithms.

The solution should meet the demands of ~300 users, 1000 messages a day, and be reliable, with low false positives.

If you are using such an excellent application to intercept incoming email I would appreciate hearing your thoughts on this.

thanks,
Alan Brown