you could use lokkit to configure the ip tables for you
its in /usr/sbin/lokkit on RH9 and you need to be root to run it
put firewall on "High" but go to "customize"
In customize you can set up the ports, and only the ports, you need open. For example, to set up port 25 (SMTP) you can:
Check off "High"
Go to Customize
In "Other Ports": type 25:tcp (Assuming SMTP port 25)
Click OK again
Note: to enter mulitple ports, separate each by a space. For example:
would open port 25 (SMTP) and port 110 (POP3)