With my 8700, I set up three email accounts. I think you have to set up more than one email (address) account, aside from the general @blackberry.net.
When I go into compose, I can hit the ALT key which shows me a list of my accounts to send from or hit the SPACE key which rotates through the various accounts. I pick the one I want to use, compose my email, and send from it.
I'm not sure if this is the info you are looking for.
7105t / 8700g