Selectively Forwarding E-mail

We're in the process of migrating some of our users to an Exchange server.
The Exchange server is going to be the main mail hub for our domains. The
Tru64 cluster will still house some users who want to retain their e-mail
addresses using the same domain name. I'm trying to find a trick that will
let me tell Sendmail how to deliver some users locally and forward others
to the Exchange server when everyone will still have the same domain name.
I've been looking at various Sendmail ruleset ideas but I haven't really
seen what I want.

In other words, let's say that I have two users [email protected]
and [email protected]. Localuser wants to continue accessing their
e-mail under Unix. Exuser wants to access their e-mail using
Exchange. Currently, if localuser wants to e-mail exuser from the Unix
server, the message is rejected because exuser isn't known to the Unix
server and has the same domain name. The other direction works because the
Exchange server is set to forward e-mail that it can't deliver for the
domain over to the Unix cluster for delivery. I think there's a way to do
the same from the Unix side but I'm afraid of creating a mail loop. 

Any ideas or suggestions on where to look for ideas are appreciated.

--
Ron Parker
Brazosport College