Relaying and Spam

Hi all,

Our company is using Exchange 5.5. Our ISP recently called our attention to our being an open relay and that they are receiving a deluge of emails from our server. We have successfully configured our Exchange to stop relaying and our ISP has confirmed that our system is now closed to relaying. However, they still say that they continue to receive spam mails from our domain. We’re trying to trace the origin of the spam mails and checking computers for viruses. But if all computers check out clean for viruses, is there some other cause for this problem aside from those mentioned above?

I hope someone can help. Thanks.