Having been created as an open-source project at the Apache Foundation, SpamAssassin seems to be one of the best spam filters available. It can be installed and configured very easily, with almost no additional fine tuning. Once installed on the mail server, the amount of incoming spam messages decreases significantly. The software can be used with many different e-mail server systems. There is already a lot of existing installations which proved SpamAssassin to be a very good choice in figthing spam.
Comments
Used it for some time
I used SA for some time - it worked not bad but still not 100%. Then I started using Gafana.com - this is a spam-blocking mail client. First of all, it's so unexpensive, almost next to free - about 30 USD per annum. Then, it is really helpful - I stopped getting spam at all and also there's no risk that someone sends mass-mail spam from my account - it has a special verification line in case some mass mail occurs..so, that's what I wanted to say about spam-blocking system that I use now.