Due to my frustration with my personal e-mail, I recently implemented some additional spam filtering which was amazingly effective.
In the past 24 hours, the system has stopped 775 spams destined to valid users, and has accepted 37 legitimate messages. Previously it would have accepted those messages. Most would end up in my spam folder to be reviewed later, but others would end up in my inbox.
This decrease in spam hitting my inbox is through Greylisting, implemented using Postgrey.
A properly implemented mail server needs to be able to deal with a temporary delivery failures, and try again later. A system implementing Greylisting will keep track of the source IP address as well as the source and destination e-mail addresses. The first time a unique combination of these three things is seen, the message is given a temporary failure for 10 minutes.
Legitimate mail is eventually delivered. Spammers give up and don't come back.