Question of Firewall & Mail Servers

[Doug Thistlethwaite <doug@dupreeinc.com>]

Hello,

I am working on adding a debian slink linux system as a fire wall to my
existing company network.  When finished, we will have an ISDN router
connected to the linux firewall machine and a separate network card
connecting the internal network to the linux system.

The mail server will be inside of the firewall and needs to receive SMTP
connections through the firewall.

My question is how is this done?  I will have fixed IP addresses for the
router and linux system, but the mailserver will have an internal
192.180... IP address.  Can this be done?  If so, where should I be
looking for information as far as setting up the linux system goes?

Thanks,

Doug