I am using RHEL 5.1 and want to deploy email solution like Scalix Messaging or Zimbra Collaborative Suite.

I email server will not be directly connected to Internet. As i have only one server in my network which shares the internet among all the machine.

So my question is that how can i configure RHEL in a way that all my external emails route through the internet sharing computer.

how can i configure this routing in RHEL ...

Any suggestions and experiences are highly needed