firewall issue? you could test with a google smart host or hotmail (if you have their accounts). Really there isn't a whole lot to IIS smtp.
check:
http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/6003ab51-7eb5-47b2-8d92-e26b1deeeac1.mspx?mfr=true
