migrating from one system to cyrus and ask sendmail configuration help

[Mike]

I'm slowly migrating from a POP3 to an cyrus-imapd solution
for my users. I have an odd setup with email that I'm leary
to touch. The box were cyrus runs has the latest sendmail that
does quasi-forwarding to a smart hub for final delivery. The box
with cyrus locally expands aliases then sends the messages to
a box outside my firewall (don't ask) running courier. Courier
delivers the messages to user accounts, then user MUAs poke
through the firewall to POP3 pull their messages.

What I need to do is to configure the local box with cyrus
to deliver *only* my messages to cyrus while delivering all
other messages to the courier box outside the firewall.

Does that make sense? When I get everything changed over I
can eliminate the wierd rules and go back to a simple sendmail
and smart hub setup.

Mike