Blacklisted in Spam Cop Need Help to Find Problem

Hi, My customer has been blacklisted in Backscatter & Spam Cop for supposedly sending spam email. I need someone to give advice: 1. Identify where the problem is (i.e. the internal network or our ISP or our Web Hosts email server) & 2. Resolve the problem. We'll limit this to 1 hour of consulting. I'm reasonably knowledgeable with networking but since this is important to get resolved I need a real expert with email systems. I need this done straight away i.e next couple of hours. My customer has about 4 pc's running win xp pro. Be good if you have Skype chat (text only). Lee.