You can run exchange best practice analyzer (exbpa), it will check for open relay. Exchange 2007 is not open relay with default setting, but if the SMTP session is succesfully authenticated, relaying is permitted.

Thank you for your response. How do you run exbpa? From the command shell?

Start Exchange Management Console and go to Toolbox (left side at bottom), you will then see Exbpa in the middle of the screen