Exchange server has a recipient filter that prevents mail submissions to accounts that are not in the GAL (Global Address List).
First, let’s locate this configuration and then we’ll talk about how it works followed by thoughts about relay domains.
How to enable this feature
To enable this filter in Exchange 2003 you would visit the Message Delivery Properties screen, and select the checkbox next to Filter recipients who are not in the Directory. You would also need to enable it on each SMTP connection.