Configuring Exchange 5.5

Configure the properties of the "Internet Mail Service". The actual configuration will depend on how your network is setup. You can lock down what machines can send, what domains are valid etc. There are some good Technet articles on the different configurations you may have.
Cheers
Glenn.

Under the Internet Mail Services properties and the Routing tab.
Click on the Routing Restrictions button.
Check the box that says "Hosts and Clients that successfully authenticate".
This will prevent mail relay form the outside.