flooded e-mail queue

I had tons of e-mails come into the exchange srver 2000, running on Win2K, with bogus addresses, and many BCC contacts. This was not relay that is off, these came in, failed, tried to send to BCC addresses, failed, and generated thousands of postmaster@domainname.com. The do not look like span and every spam suite I have found does not check the BCC field. My mail is passed through my ISP, to a Linux firewall, it in turn passes it to the excahnge server. I was able to stop but filtering the IP address on any protocol and stopping the inbound mail mess. Is there any way to check for valid users for inbound mail? Hardware, software?