I am getting more and more confused :)
If the sender is a NATed box in 192.168/16 space, and the receiver also is a NATed box
in 192.168/16, rhe received message will have a by 192.168.xx.yy, and seemingly never
left the trusted network.
Ahh, but this can never happen over the open internet. When the NATed sender sends mail to your NATed server, the server will not see the mail as coming from 192.168/16. It will see the sender's public, post-nat IP.
In order to NAT, it actually has to be translated by the NAT before it can hit the internet. Anything which has nothing but reserved IPs, by definition, must never have been on the internet, as it never got translated to a public IP at any point.
ie: xan.evi-inc.com is NATed. If it were to send mail to this list, you would not see it's private IP, 192.168.50.6, as the server dropping off to apache.org. You'd see 208.39.141.86.
ie: Received: from xan.evitechnology.com (HELO xanadu.evi-inc.com) (208.39.141.86) by apache.org (qpsmtpd/0.28) with ESMTP; Thu, 02 Dec 2004 13:01:32 -0800
