Overview

Mail sent to a domain hosted by your account that uses third-party MX records does not arrive to the intended recipient.
Mail is instead delivered locally to the server that hosts your domain.

Cause

The domain is active in Mail > Mail Routing within the control panel. Any domain listed will bypass the MX record lookups and gets delivered Locally.

Solution

In Mail > Mail Routing within the ControlPanel the Domain can be de-authorized/removed. This will prevent the server from handling mail for that domain.