IMAP and SMTP port redirect

[Arun Shrimali]

Hi everyone,

I have some network related problem.

I have network as follows :

LAN (172.16.251.0/255.255.255.0 ) ---> Linux server (fedora 6, eth1
172.16.251.234, eth2 172.16.250.246, gw 172.16.250.245) <--NAT on
router--> router (172.16.250.245)---> internet

In LAN, PCs are not routed through gateway, because of security, I use
squid for HTTP proxy using NCSA authentication.
Now, I need configure Linux server to REDIRECT 143 port and 25 to wan.

I would like to set LAN-PCs email clients imap server as:
172.16.251.234, when email client would like to download mail, he
asked 172.16.251.234:143 and 172.16.251.234 will send packets to
remote imap server (everyone in LAN use same imap).
For smtp is the same case.

Please, advice me, how to set iptables chains on linux machine.

I have google a lot, but lot has confused me a lot

Can anyone provide me a simple solution.

Thank you very much and regards

Arun