I have gmail, it works fine, as far as reading/writing/replying goes. You give it your account info and everything just works (it syncs your contacts with the People hub too). If you want to, like, manage your account from the phone, or generally do anything other than reading/writing/replying, you're mostly out of luck as far as I know. (well, you can delete messages and move them between folders, but you can't create folders or fancy rules or anything).