even if you're on a BES you can still configure additional email accounts to send mail to your BB. This is done with the blackberry web client - the user creates an account online where his address will be (for instance)
and sets up his other POP3 accounts to send email to his blackberry. Wiping the handheld won't work - you probably need to have the SIM card re-provisioned or get a new SIM card.
Hope this helps!