ISA, Mailrealay and DMZ issues...

I am trying to configure our email to go through our DMZ which currently
has a mail relay server, and I?m having a hard time getting email to go
out. This is how I would like it configured.

ISA SERVER = Trihomed   Mail Relay = 192.168.XX.XX  Exchange = 10.80.XX.XX
Public = 63.68.XX.XX
Private = 10.80.XX.XX
DMZ = 192.168.XX.XX

Email comes in and hits the ISA server (63.68.xx.xx) which has a SMTP
protocol rule pointing all incoming email to the Mail Relay server
(192.168.xx.xx). The Mail Relay Server than has a Remote Virtual SMTP
Server established on IIS to point all incoming email to the Smart Host
which is exchange(10.80.xx.xx). This configuration works fine for incoming
email, but I can?t seem to get things to work for outgoing email. When an
internal users tries to send email, it first hits the exchange box, which
has a SMTP connector  pointing to the Mail Relay. The problem is that the
mail relay server wants to send all incoming email to the exchange server,
this obviously becomes a routing problem. If I tell the exchange server to
use DNS instead of putting in the Mail relay server?s IP address, the
email goes out but avoids the mail relay all together, which defeats the
purpose of what I?m trying to do.

I have read a few articles and many of them mention the mail relay has to
have a public address, but this then opens up the Mail relay server for

Thanks in advance.

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