Download startupitems for ipfw and move it to /Library/StartupItems/.

Create a textfile called "firewall.conf" and move it to /etc/firewall.conf

En example firewall.conf is located here, shows China and Korea* blocked for port 25,

to prevent junk mail from reaching the mail server.

Make sure that Firewall stays turned off in Apple's Server Settings.app.

* Source: http://www.okean.com/sinokorea.html