Remove the Authoritative Feature for your Domain

In your Control Panel
Click on the name of your Hosting Plan (typically Exchange Hosting, SharePoint Hosting, etc)
Click Hosted Organizations
Click your domain name
Click Domain Names
Select the built-in domain as the Default Domain, then click "Set Default Domain"
(the built-in domain is typically in the form of: domain.myhostedsolution.net)
Click the red X next to your domain.
It may take up to 5 minutes for this setting to take effect.