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.
For example, if
example.comuses Google Apps to handle e-mail, and mail originates from the same hosting server as in from a script, like a contact form on a WordPress blog, mail would not reach its intended recipient at
firstname.lastname@example.org, which is accessed through Google Mail. Mail, instead, would be delivered to the local user named
contacton the server.
The domain is present in Mail > Mail Routing within the control panel. Any domain present in this list will bypass MX record lookups and the server will act as its final destination.
Visit Mail > Mail Routing within the control panel and deauthorize the server from handling mail for that domain by deleting the record.