Not sure if this is the right spot to post this, but I have setup Google Apps email for one of our domains.
I can receive and send messages from domains that are not hosted on the same server as the one we just setup Apps for.
For example lets say I setup Google Apps for test.com. Example.com is also hosted on the same server. test.com will receive emails through google apps from a.com, b.com, c.com, etc. that are hosted on other servers. test.com will not get emails from example.com that is hosted on the same server.
It is a plesk sever. For some reason it will intercept the emails instead of letting them go to google. I can see them through the plesk webmail. Apps will receive emails from domains not hosted on our server.
The MX records are exactly as specified in the docs.