who i set up MX in DMZ

i am implemeting exchange 2003 and ISA server.

my senerio is:

i have three zone.
Exchange server ISA Server with three LAN Card, one is exteral seconed is internal and third one is public ip.

my iSA Ip are:
10.0.0.1:255.0.0.0 (DMZ)
172.16.1.1:255.255.255.240 (Internal)
one is public ip.

in DMZ i use exchange and with his ADS and DNS
i want to publish it to ISA server.
in exchange i have no live ip.
where i configure MX record from email.and what is the dns setting of my DNS Server.
what rules i make for to access it trough ISA to my internal and external client.

Note: i register my name server only against my domain.