MS Exchange relay problem

Here is the question.
I have a client using MS-Exchange Server 5.5, and he keep having unsolicited commercial/porno email relaying in the server queue. Hence the server queue keep getting build up and delaying the server’s users mail from sending. I have try using routing restriction wif their internal IP addresses in the “NEVER route with this IP addresses” option. However I still get alot of UCE mail realy. Can anyone help me on that issue?