Thanks everyone! With your help I was able to find the offending domain (fuzzydragonmartialartssupplies.com) and the client was running OSCommerce. With just a small amount of research I was able to find that OSCommerce was susceptable to spammers. We have addressed this issue and hopefully this will no longer be a problem. Thanks again, you guys are the best!
Hello. My server (188.8.131.52) pepperjack.pepperjack.com has been added to the blocked list today. As suggested I went to the site http://psbl.surriel.com and found that the problem seems to be spamtrap mail. If one of the clients on my box is sending spam I will certainly get rid of them. Our smtp server does require login before sending or receiving email, so hopefully I can track the person and identify which domain it is coming from. Where should I begin? I am new to spam tracking.