DMZ/mailserver topology question

I am doing some redesign and want to replace the current Exchange 5.5
relay currently in our DMZ with a *NIX box running Exim (for economic
and security reasons). We have an internal Exchange machine that keeps
all the real mail data.

But then I was thinking, I run a Linux box as my interior
router/firewall--should i just port forward the mail requests to the
internal Exchange server? What kind of security risk does that
present? Which is a better solution do you think?

TIA.

Bill