IP masq and Firewall

["Durham, Kenneth J" <KJDurham corp earthlink net>]

I have recently installed IP masq and Firewall on a redhat 6.0 system.  I
have 7 computers routing out of the IP masq/Firewalll box.  I wanted to know
if there is a way to hook up one of the machines in the internal network
behind the firewall?  I have IP masq setup on machine A with an IP of
192.168.0.1 then computer B (the one i want to be a mail server) 192.168.0.2
I want to be able to get mail from outside the interlnal network as well.  I
have my  own domain as well.  I just dont know how to setup this kind of
configuration.  If anyone needs anymore info please do email me.  Thank you
for your time.