exchange 2000 spam

help! my exchange server is getting hammered by spam and i can’t stop it. the server is sitting behind a d-link dfl-300 firewall that is only passing port 25 traffic through. i’ve stopped the default smtp server and tried creating a new one. no sooner does it start, there’s 5 or 6 “users” in the current session and a thousand messages in the queue. none of these are actual users and i have relaying turned off, regardless if they can authenticate or not. it seems like there’s something running on the server that allows this. a virus maybe? norton enterprise av client as well is norton for ms exchange is running on the server. windows 2000 sp3, exchange 2000 sp3.