Email troubles behind iptables firewall

["Travis Crook" <travis@visionsbeyond.com>]

Hi all,
    I have an iptables firewall setup to route web, ftp 
and email traffic to internal servers.  My internal 
servers have ip addresses in the 192.168.2. range.  
The problem I am having is that my email server is 
being used as a spam box (not by me but by someone 
else on the internet - not with my approval either).

My question is this:  Qmail is my mail server.  It is 
supposed to only allow relaying by certain clients.  
As far as I know it is set up this way.  I think the 
problem occurs because I allow 192.168.2. ip 
addresses to relay mail (i.e. everyone behind the 
firewall).  But every email coming in to the mail 
server gets a 192.168.2. ip address.  Is this 
correct?  Is there any way to pass their real ip 
address through the firewall to the mail server?

Any help would be greatly appreciated.  I can 
post the firewall script, etc. if needed.

Thanks

Travis Crook
Visions Beyond