If you have cleared the device and still do not have the option to set the external email, then I am 99% sure that it is the provisioning, now that you have wiped the device, you can go back to the T-Mobile monkeys with ammunition and say that it can't be your BES as you wiped the device. Give them a call and tell them to check you have been provisioned, as far as I remember, you should have 3 services provisioned by them so that it will work.
M@