How can I configure Spamassassin to filter all the incoming mail on my server?

Spamassassin is a powerful spam detection system that uses a variety of methods to identify spam in incoming mail. Using Spamassassin in combination with the Mail Deliver Agent (MDA) Procmail, it can filter mail identified as spam in different ways. For example, there is the choice to delete it, put it in a special folder, or change the subject line to indicate that it is spam. Setting up Spam...