There is a setting that can be set by default to stop relaying of any kind and that is under the outbound connections settings:
If you go into the properties of your SMTP server in IIS6 Admin:
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And then into the delivery tab at the box, and find the
Outbound Connections button at the bottom
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Check that the number of connections has not been set to 0, as this will prevent all outgoing traffic.
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If you set it to 200 and make sure you have set who can relay etc. to only internal addresses, you will have a functioning SMTP server.
Make sure it is secure before you do this - check that the queue has nothing unsavoury in it first under c:\inetpub\mailroot\queue\