RBL ZA

The first place I would suggest you start is visiting the website http://www.dnsstuff.com.

There is a "Spam Database Lookup" option which you can enter your SMTP servers IP address in to and search various RBL Databases.

Here are some more options for you:

http://www.mob.net/~ted/tools/rbl.php3

http://www.aboutmyip.com/AboutMyXApp/BlackListCheck.jsp

If your IP address is listed, you will need to follow the instructions of that particular RBL list to be removed.