How to add Postini subnet firewall and restricting smtp and port 25

i have recently signed up for postini services. As part of setting up the service i need to add postini subnet 200.123.0.0/20 or 255.255.240.0 and restrict smtp traffic and port 25 access to this subnet on my firewall

i have cisco ASA5510 firewall and exchange server with internall ip address 192.161.2.1 and an external ip address of 2.2.2.2

what commands can i run on my firewall that will give smtp and port 25 to postini subnet 200.123.0.0/20 or 255.255.240.0 and ensure that emails are still delivered to the exhcange server?

PLease if you could give the configurations commands that would be excellent.