You need to see the list of all valid recipient domains for Exchange 2003.
You can do this via System Manager on the Exchange server.
- Run System Manager
- Click on <ORGNAME> (Exchange)->Recipients->Recipient Policies
- You will see the policies. Most servers will just have Default Policy.
- Double click Default Policy.
- Click on the EMail Addresses (Policy) tab
- You will the list of valid DNS domain names that Exchange will accept mail for.