Configure Exchange 2000 to stop intruders to log on via SMTP virtual server

We do run Microsoft Exchange 2000 Server on Windows 2000 serer. We noticed that intruders (spamers, etc) log on to the SMTP virtual server regularly without authorization for even long periods of time. Our ISP claims that our Exchange Server is hijacked to send spam email. Deliverables: a) Identify whether our Exchange Server is hijacked to send spam emails b) Identify and close the loopholes thru proper configuration c) Identify the cause for? unauthorized individuals to log on to the SMTP virtual server and stop this thru proper configuration ## Deliverables d) Deliever a brief description of what was not configured well and what of the configuration was changed? how ? to remedy the issues.