First, you need to create 2 Address Objects. one which corresponds to the public IP of the Exchange 2007 server and one that corresponds to the private IP.
For NAT policy, you have to add 2 rules that are mirror images of one another. Go to the WAN > LAN area. Add first NAT policy:
Source Original = Any
Translated = Original
Destination Original = Public IP address object
Translated = Private IP address object
Service Original = SMTP
Translated = Original
Interface Inbound = WAN IP (X1 interface)
Outbound = Any
Add second NAT policy:
Source Original = Private IP address object
Translated = Private IP address object
Destination Original = Any
Translated = Original
Service Original = Any
Translated = Original
Interface Inbound = LAN IP (X0 interface)
Outbound = WAN IP (X1 interface)
Then you need to add a firewall access rule:
Source = Any
Destination = Public IP
Service = SMTP
Action = Allow
Users = All