Have you thought about using a WAP webmail sevice like Gmail? All you need is a cheap, low-speed web connection (like T-MobileWeb for $5.99/mo unlimited). I think doing that would finesse the network interoperability issues, since email is the messaging lingua franca of the internet.

It's not in the contract, but mine works in Europe without data charges if I make sure the phone registers on a T-Mobile site by manually selecting "Network."