Anti mail-relay for IIS 4.0 running on WinNT4 Server

I have the a WinNT4 server installed with IIS4.0 running WWW, FTP and SMTP services.
They are all needed, which means turning it off is not an option.

The server is co-located at an ISPs data centre.

My question is related to the SMTP services.
I have already set the option not to relay mail, in the IIS Manager.
However, I can see that there are hundreds of mails being pumped out by my server from the queue.

I am running the latest version of TrendMicro OSCE 6.5 with patterns updated. No viruses/worms found.

I suspect the server is being used as a mail and spam relay.

How can I confirm this, and how can I stop it?
wlseetAsked:
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wlseetAuthor Commented:
I mean hundreds of mail every minute.
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Paul SDesktop Support Manager / Network AdministratorCommented:
turn on authentication for all smtp clients. send out an upate ahead of time to all clients. then on a specific date switch over. this forces only registered users to be abled to send out mail. it also makes it easier to see who is sending all the email.
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Paul SDesktop Support Manager / Network AdministratorCommented:
check your logs and find the ip address. try running netstat to get the culprit ip address. then block the address from the router or firewall.
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