How do you do the address mapping, I need to ensure our exchange server is presented with the same public IP each time. At the moment it's randomly choosing one of our public IP's!!

Thanks

Uncheck the 'join NAT IP Pool' from your additional addresses under the IP alias section. This will assign all outgoing traffic to the single WAN IP address.

If you need it to use a different IP from the main WAN then use the address mapping features and specify that local address to use a particular WAN address.

Of course, if you wanted direct access to that server you could always create an IP ROUTED subnet and have that IP address configured directly on the NIC of the server.