Not sure if you have been blacklisted? You can check out the following: Blacklist Monitor, Return Path's Sender Score, and Habeas' RepCheck. Some of the blacklists are small and don't really matter. Stefan states that you should be concerned if you find yourself on any of the following lists: Spamhaus, SURBL, or SpamCop. The important item to keep in mind is to find why you are being blocked and to cut out whatever is causing it.