Not a good idea to use the carrier supplied address:
1. Totally unfiltered and if it gets out, you will get more SPAM than you want.
2. You can't take that address with you when and if you change carriers. Once the account is closed, good bye email address.
3. The only way to read email on that account is via the BB. There is no webmail option and in the event of a data outage (and they do occur) you are without email until it comes back up. And the chances of lost email increases.
Why do you say the device cannot use BIS? What error message are you getting? Perhaps the user has already set up one?