Win 2000 Adv Serv and IMail email server and someone is sending all this spam through my server.

There is this funny thing happening, I have Windows 2000 Adv Serv with all the latest updates, NAV Corp 9.0 installed and IMail server (one of many services) installed on my server and from few weeks ago I have noticed in my queue of outgoing msgs that some of the msgs are being send from my server as a spam to random people with spam info in them. I have performed complete system scan with NAV, I have check for spy ware with two programs and found nothing so the msgs are still being sent. I have set relay setting to "Relay for local hosts only" and that always worked for me, but now somehow someone is able to send stuff by my SMTP service. I tried to send something and it does not allow me to do that, so I’m assuming that my relay setting is correct. Would anybody have any idea what could that be or how is that possible? I would greatly appreciate any advice. Cheers.

Lucas

PS. If you have any questions, i would be more than happy to answer them for you. Thanks.