Configuration of local mailserver behind linux firewall - HELP!

[doug@dupreeinc.com]

Hello,

I have a linux (2.2.X kernel) box that I am using as a firewall between
my internal network and our new ADSL modem.  I am currently working on
configuring the ipchains on the linux box to protect my internal
systems.

Question: My internal mailserver is currently running on a HP-UX C3000
which
is behind the firewall (IP: 192.168.XXX.XXX).  I need to configure my
linux firewall to pass the SMTP transmissions to and from my server.

What is the best way of doing this?

Are there any additional pitfalls I should be aware of to make my
systems as secure as possible?

Thank you for your time! 
Doug Thistlethwaite