By default the Exchange 5.5 installation allows open relay through the server. Prior to SP3 for Exchange 5.5 this could be fixed only through changes in the registry. After applying SP# or SP4 for Exchange 5.5 there is an additional option in the Routing TAB of Internet Mail Service - Routing Restrictions. Here you can see how you can disable Open Relay through routing restrictions.
I just finished the "Upgrade from Exchange 2007 to 2013" Part I screencast and must tell you that it is extremely well done! I found no variances between the video/script and what I experienced. At the end of Part 1 ...
Bob Duffett, Priority Software Inc. ...more...