Nat question - Interface required for NAT?

[Geraldo Macedo]

I'm sending this again under a new subject, as the first may not called 
for attention.

Greetings from Rio de Janeiro,

I'm designing a firewall for  client that has the following configuration:

         Internet
             |
             |
         Router Internet
             |
             |
         Netfilter Firewall
             |
             |
         Router Intranet
             |
             |
         Client's LAN

My client has a mail server running QMAIL at his LAN, and he wants to 
use it as his Internet Mail server, without changing it's place on the 
network.

My question is: Can I make a Destination NAT (for packets from the 
Internet) to an address that I don't have an interface?  Or do I need to 
make first a translation at Router Internet and then another at the 
Firewall?

I know that I can use something link smtpd on my firewall to forward all 
mail to the QMAIL server, but I'm hunting for options.

Best Regards,

-- 
Geraldo Macedo		| Quality Software
Unix Administrator	| +55 21 34753009
glmacedo@quality.com.br 
| Rio de Janeiro/Brazil