I am still very new to Linux and could use some help locking down the server so that it will only accept email from the filtering service. I have in my iptables file

:RH-Firewall-1-INPUT - [0:0]
-A INPUT -s xxx.xxx.xxx.0/ -p tcp -m tcp --dport 25 -j ACCEPT (the filtering service ip address for their server)
-A INPUT -j RH-Firewall-1-INPUT

-A RH-Firewall-1-INPUT -p tcp -m state --state NEW -m tcp --dport 25 -j DROP

I am trying to lock down all incoming email if it doesn't come through the filtering service. Any help in this matter would be greatly appreciated. Thanks.