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.

Step 1 Click here to watch the Video …