[CentOS] problem sending/recieving mails

Hi all,

I ave setup a CentOS server to act as LAN gateway and also as a 
transparent proxy server but all client "passing through" that server 
are enable to send or recieve mails.

The mail server is host on the same LAN running mdaemon, both servers 
are on private IP block(192.168.0.0/24). Am using cisco router to do 
port forwading for mailserver(25,110,143) ports.

If i eliminate CentOS box and direct traffic direct to cisco router 
everything works.

My question is why is it when i use CentOS box as LAN gateway clients 
are unable to send/receive mails? I ave done nating on that box.

Centos 5.5 full updated.