Hi,
Do you have any restrictions set on Exchange as to what IP addresses can communicate with it?
In the web UI, through Scanners (left hand system menu) and then click the scanner name followed by Configure Scanner. On the SMTP tab, under Outbound Mail Settings, have you allowed Exchange to recieve mail from the IP bound to the interface shown there?
It's been a while (long while) since I looked at Exchange 2000 but I think you can restrict connecting IPs via the IIS SMTP properties.
Let me know if that helps.
--ian