Yes, you're making perfect sense and I can understand your frustration but bear with us.
The problem is your NAT device and the need to resolve your IP vai DNS. Because you're behind NAT you need a local DNS server, if you don't have it then you'll need to install BIND and follow the instructions in the wiki on
split-DNS.
Just as an aside, you check your settings with the following commands:
Code:
dig yourdomain.com any
dig yourdomain.com mx
host `hostname`
those three commands will get you enough information to check your set-up. Are you going to use Ubuntu for this install?