the BES is just an add-on for Microsoft Exchange or Lotus Domino. if you have one of these, then you can have the various mailboxes forwarded to coinciding mailboxes in one of those environments and the BES will redirect as instructed by its setup with those two environments.
so if you had
setup on his server and
setup in and Exchange environment, and BES was setup with the exchange environment and "John Doe" was a BES user, then it would work. you could have a static mailbox with forwarding setup on the POP3 server or a simple alias that is setup to forward anything addressed to
to have an end delivery to
(so no mailbox needed on the unix side - uses virtual user tables in sendmail).