I have a client who runs a SBS 2003 SP2 with exchange server 2003 SP2(they have 1 server). They are having problem with mail server being flooded with spam. It sits behind a firewall which has port 25 redirected to the server. After we got the first flow of spam, we applied filters on recipient, sender amd connection(in the properties of Virtual SMTP). We renamed the queue folder and restarted with a new queue, it worked fine for a day and we restarted getting another flow of spam (sender: email@example.com). we recently updated our internet security on all workstations and server and did a full scan ( everything came up clean). What coulfd be the problem, is there a way to trace the orogin of the spam? Any setting that i could check ?
By the way the server is not an open relay.