In this case IPtables can be used to retrieve mails.
A model Network setup:
Eth0 : Public N/W
Eth1 : Private N/W
IP Range : 192.168.10.0/24
So hence the Iptables rules for forwarding the packets ,
# iptables --table nat --append POSTROUTING -s 192.168.10.0/24 --out-interface eth1 -j MASQUERADE
# iptables --append FORWARD -s 192.168.10.0/24 --in-interface eth0 -j ACCEPT
# iptables -A INPUT -s 192.168.10.0/24 -j ACCEPT
Insert the above rules,
Save and restart the service
#service iptables save
# service iptables restart
Now you can download your mails in the Outlook express.
Client Side Configuration:
1. Set the Gateway address as Eth1 IP.
2. Open Outlook Express – Tools – Accounts – mail – add new
3. Configure your mail account.