Blocking External Email w/ Exchange 5.0

Yes. My last job included duties as an Exchange 5.5 admin, and I don't have an Exchange box to play on anymore, so my answer is from memory. Hopefully my memory will be close enough to the way Exchange 5.0 works to get you what you need.

In Exchange Administrator, go into the Connections container, select the Internet Mail Connector, and click the Properties button.

Look for a tab called Connections. It should have a list for Inbound and Outbound connections, and each list should have the options of "Only the list below" and "All except the list below."

You can put IP addresses, ranges of IP addresses, or domain names into this list. Then you can check the "all except the list below" button to block incoming mail from those unwanted domains.

Good fortune,
Don