Not applicable. He is on T-Mobile.
It should work on T-Mobile seamlessly. You add email account to your email setup on T-Mobile Web site, someone emails you to that address, when you reply your blackberry is supposed to sign your message with that address, not
You may have to contact customer support, or check your configuration. The only case when it is not supposed to work that I can imagine is when you have something like Google account, and you configured it through Google configuration web page to forward emails to
In that case neither T-Mobile nor Blackberry know any other address of yours, but
- and use it to sign replies.