"554 Relay rejected for policy reasons" error

I'm trying to add a second Internet domain to my server for Web and email addressing. My ISP has pointed the DNS Zone file and MX record to our server IP. I've created a Virtual Host record on the Domino server and the domain web site (e.g., "http://newdomain.com") comes up okay. And I created a Person record in the NAB with mail address in the new domain (e.g., "john@newdomain.com"). But I get "554 Relay rejected for policy reasons" error message when trying to email to this above address.

What am I doing wrong?

You need to put the domain in your Global Domain Document as an alternate domain.