How do I handle email forwarded to a subdomain, but deliver to the user in the parent domain?

Here is the situation:

I have a unix mail system, running sendmail, that handles all inbound email for the domain school.edu. Most of the users, namely students, have home directories on the unix system, and use squirrel mail or pop3 to check their email, so sendmail performs local delivery of the messages on the unix system. For faculty and staff, I want to use exchange 2007. I currently use a .forward file, or an entry in the aliases file on unix to forward faculty and staff email accounts to username@subdomain.schoool.edu, which is the smtp agent for our current groupwise system, and GW dutifully delivers the message to username@school.edu. In other words, the groupwise system automatically equates username@subdomain.schoo.edu with username@school.edu.

I have tried to setup a similar arrangement with exchange 2007, but exchange seems to be pickier than groupwise about the fact that username@subdomain.school.edu is NOT the same as username@school.edu.

Any ideas for what I need to do in/with exchange routing to make this work? Unfortunately, the unix side of things is pretty inflexible, in that we aren't allowed to change folks email addresses, and we are trying to make this as streamlined as possible.