Unknown Mail Received using our domain - How to trace?

Hi guys, One of our user receives mail from unknown user from which it used our domain name. I've check its ip address from 206.142.244.24. using the kmail viel all headers. I've check in our server (mail server and internet server) no user exist. Even the nslookup doesn't know the DNS. Can you help me guys how to check the source of this mail? Thanks -- Jonathan J. Terbio = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =