> firewall) is 192.168.0.33. Clearly, trying to reverse-DNS the latter will
> lead to trouble, whereas the former is OK.
You are using NAT - if you only want to send email from your internal
network to the world and get your mails by "polling" it somehow you get no
problem (except that your netblock may be blocked by some mail servers).
If you want to provide services like smtp to the world you have two
choices:
a) establish that service on your NAT box (I assume it's impossible on
that $155 box)
b) your box must be able to redirect defined ports to hosts at your
internal network. Most NAT devices can do that, some cannot.
Regarding DNS:
If you provide services to the world always the address of your firewall
box is visible to the world. Your internal addresses don't matter.
Regards, Frank