iptables

iptables -F                                                    -F: flush
iptables -X                                                    -X: delete-chain

iptables -P INPUT ACCEPT                                       -P: policy
iptables -P OUTPUT ACCEPT
iptables -P FORWARD ACCEPT

iptables -A INPUT -s 127.0.0.1 -p tcp --dport 25 -j ACCEPT     -A: append
iptables -A INPUT -p tcp --dport 25 -j DROP