MS Exchange Relay Security

I am running Exchange Server 2000 behind a firewall, with a 192.168.X.X subnet. Right now, some spammers are using my machine to relay their email out to people. Of course, I do not like this.

I tried setting the Relay options in my SMTP serverto only allow 192.168.X.X to relay, but then all mail sent TO me bounces with the message that my server is unable to relay for my static IP. Does this make sense?

So what can I do? I want to set up so that computers on my subnet can send/receive mail, but that no one else can use my machine to send mail.