If you are having trouble setting up your @cox.net email and can receive email but are NOT able to send them....don't bother calling T-mobile they have no clue. After 2 calls and 2 hours they told me that it won't work with the MT4G and its not their fault.
That said, cox had the answer in minutes. On the outgoing server settings you need to 1.) check the box login required 2.) pick SSL in the security drop down box and 3.) make sure that the server port is 465.
It worked for me. Thanks Tmob for terrible customer service!
As for Gmail......worked right out of the box.