spam in Exchange Server 5.5

Dear Fellows,

I was invaded for a spam stream and it?s consuming too much resource in my network. The PDC and the BDC (which is also my mailserver), are running Backoffice Server 4.5. Now I really don?t know how to extinguish this spam from my Exchange Server 5.5. I have no firewall, only MS Proxy installed in my PDC.
Any ideas? Is there any application which can protect me against other future spams?

Thank you very much

Paulo