sendmail and firewall

Hi ,

I had my linux running sendmail as mailerver .
I also had 2 network card installed on that machine ,
one with private ip connected to lan and
other one with virutal ip connect to internet , which means linux
acts as simple router .
My question is that I want to install a firewall and
let my mailserver connected to my lan only ,and
then forward all packets to internal interface of my firewall .

I'm not sure how and what modifications should be made to my current
linux configuration.

I thought I have to disable the network card with virtual ip ,
change default gateway . Is that all I should do ?


Can anybody help me ?
Any messages from you will be appreciated .

best regards