Danny,

We have found that most companies use NATing and therefore, the outside IP address is always the same regardless of what they use on their LAN. This is obviously not the case on your network.

The second IP address in the header should not matter. In fact, even the first one should not matter. I say that because the receiving email server does not look at the email header to determine the source - they look at the TCP/IP header. Email headers can be forged very easily but TCP/IP headers are not very easy to forge, particularly if the message is coming from the Internet.