Spam filtering with Postfix

UPDATE 2019: the link below is unfortunately dead.

I’m moving from Exim to Postfix, most of all because of the simplicity of Postfix’ configuration files. Most HOWTOs about spam filtering with Postfix combine it with virus-scanning and other helper software like Amavis. All I wanted was plain spam filtering - perhaps virus-scanning will be added later, but for now i want simplicity. The document Postfix and Spamassassin describes this in just the way I wanted.