Postfix in DMZ ?

Here's what I want to do, I'm just not sure about the best solution.

We have IPCop as the gateway server (doing port forwarding and
maintaining 2 VPNs).
Behind it, we have 1 Postfix server w/Courier IMAP and 1 Exchange 5.5
Server (soon to be moved to another Postfix server).

I want to put a mail server in the DMZ, whose sole purpose will be to:
1) authenticate all users for SMTP w/SSL;
2) accept all incoming mail and only relay that mail for internal
recipients to the appropriate recipient.

I think that's it for now.

So, if I put postfix and Courier IMAP on that server, as well as all
accounts and passwords, would that be an appropriate solution?

Also, I'm a little confused as to how a mail server in the DMZ would
route mail to the internal network. Or is that something it wouldn't
have to worry about, but IPCop would (since it's the router).

Thanks for any input.

Curtis