SMTP Relay

I have inherited an email system that I am having trouble with. They have Exchange 5.5 set up on there LAN. All mail is forwrded to an SMTP server, running IIS 4.0, that sits outside of there LAN which is used to forward messages for the Exchange server. I am now receiving emails saying that this machine is forwarding Spam. Sure enough, the Mailroot\Queue is full of junk mail. As I am unfamiliar with IIS, how can I restrict this from happening. The IIS machine has the latest service packs. The Eschange Server does to, although the machine itself has only service pack 5. Any help appreciated.