This page includes a lot of links that will help you determine if the virus scanners you run are working or if you are on a RBL or other black list. It also shows the Top 25 spam sources found whether you have an open relay or a plan to get rid of spam forever.
It is very important to secure your mail server and make sure it is not sending un authorized mails or being listed in RBL black lists.
Test your virus scanners that they are working correctly
This test sends an email with the EICAR virus test file. This is not a real virus but a test that triggers all popular virus scanners. The site will send the test file in an email to the address you enter.
Are you listed on a RBL list?
You may not even know that you're on a spam relay. It takes a bit of getting used to, but the site effectively inspects a lot of RBLs.
Spamhaus Top 50's:
This contains a list of the 50 worst spam sources.
Do you have an open mail relay?
This test checks for an open relay on your server. If one or two of the tests fail, then you may have a protected relay. The test should locate it while attempting to relay within your domain.
Paul Graham A good plan for Spam:
This page contains useful and relevant information about how the Bayesian spam filter works.