Ok, I am assuming that you are using access-lists not conduits and you have more than one public IP address from your ISP (not port forwarding on a single public ip).
Lets say your public IP address you're using for your mail server is 65.12.32.2 and the inside (real) IP address for your mail server is 192.168.1.2. In the senerio I just described, you have something like this in your config:
static (inside,outside) 65.12.32.2 192.168.1.2 netmask 255.255.255.255
access-list outside_in permit tcp any host 65.12.32.2 eq 25
access-group outside_in in interface outside
In this case we are allowing smtp (tcp 25) from anywhere on the internet to 65.12.32.2 (our mail server's public IP address). You stated that you only want to allow mail traffic from 64.18.0.0/20. All you would have to do in this case add the following access-list:
access-list outside_in permit tcp 64.18.0.0 255.255.240.0 host 65.12.32.2 eq 25
Then remove the old access-list and you're done! Your new config will have the followig entries:
static (inside,outside) 65.12.32.2 192.168.1.2 netmask 255.255.255.255
access-list outside_in permit tcp 64.18.0.0 255.255.240.0 host 65.12.32.2 eq 25
access-group outside_in in interface outside
Please let me know if this helps
Bryan
