How can I relay mail to multiple SMTP servers?

How can I relay mail to multiple SMTP servers?

We want to use Google Apps for Domains as our email service but the
"corporate" office want's the emails in Exchange. I would like to set up a
relay (is that the proper term) to send any incoming mail to both, Google
and Exchange. That way, we can use Gmail for our daily communication (both
sending & receiving) and the "corporate" people are happy getting a copy
of our incoming stuff.
Or is this completely insane? :)
Thanx!