I have a Cent OS dedicated virtual server with Postfix. I also have google apps on a local domain.
When using any local applications, like PHP, to send email it will try to look up the host locally and deliver the mail internally rather than utilizing the google apps domain.
For example. A php script utilizes mail to send to localdomain.com. Postfix sees that localdomain.com is hosted on the same server. However, localdomain.com is hosted at google apps. Postfix will return an error saying 'unknown user'.