Spammers using mail domain name

I have received complaints the spam is being received from a company email address Postmaster@domain.co.xx (No mail account exists of that name on the domain)
Someone has stolen the domain name and used it to promote their mortages etc. They are using the external ip of the router which leads me to believe that it is from a dns server they have obtained the domain name.

This domain is a Brokers firm and this is causing a lot of turmoil. I am at a loss as to how to stop this has anyone gotthe answer.

Exchange 5.5 on win2000 server through a router w/ firewall
The war on spam has just got personal!