I am a little further thanks to everyones help, this is the current status.
Mail setup, I have the following configured and working, but what next?
A 2Mb ADSL connection into a Netgear DG834 router in straight through mode (Public IP on both the WAN & LAN side of router) then into one of the NICs on the Xserve (with a different Public IP) and other NIC (Private IP range) into a Gigabit switch.
I have local DNS, Firewall and NAT running on the Xserve and it is setup as an OD Master.
I want to run the mail server and my questions are -
1. Do I need to change anything on the router or just open up the Firewall?
2. Do I inform the ISP controlling our domain to point the MX record to the router Public IP or the Xserve one Public IP?
If anyone has any other comments I am willing to listen and learn.
Pippa.