The common name should be the external owa2.sbcity.org address.
Fm005 | Apr 8 2009 4:56PM GMT
The common name can be any of the names that you need to secure. Generally it will be the main Webmail address used by users in your organization as this is the name that will show up on the certificate if someone were to try and view the actual certificate.
But as far as it goes, there is no difference in functionality on which names shows up as the common name.