hello, please help me with this trouble..

I have a email server behind a firewall with linux , and i want to configure the firewall to pass trafic between internet and email server and email server to internet. the firewall have two nics, one nic have public ip and the other have a private ip and it's conected to the email server nic with a private ip too. how can i do that?, do i need to use nat table of iptables for something or what ?

u need to know iptables commands. visit www.netfilter.org to learn it.

i will try to explain DNAT;
DNAT changes the destination ip address from ip packets headers. so it can be forwarded to host which is located in private lan. SNAT must be enabled for that private host. otherwise ip packets comes fron internet cannot return. so connection cant be established. sure first u must turn on routing in your linux.

example.. eth0 has public ip of email server. eth1 is connected to email server. email
public ip : 212.1.1.2
server ip : 192.168.0.2

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 25 -j DNAT --to 192.168.0.2
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 110 -j DNAT --to 192.168.0.2
iptables -t nat -A POSTROUTING -s 192.168.0.2 -j SNAT -to 212.1.1.2