surely they are the same person!
whoever owned it last and setup email on the device, you need to call them and ask them to call their carrier and have their BIS account deleted.

If they have a BB and use their BIS, then they should have used the change device function, so that the email addresses work on their BB, which would have in turn released the PIN of your device.

Can you register you device for a BIS account on your carriers BIS site or do you get an error?