Quote:
There are often legitimate non-routable, or non-DNSable IP's in an e-mail header. In particular, there are often private IP's in the 10.x.x.x, 172.16-31.x.x, or 192.168.x.x ranges. There may be other IP's that are in someone's internal system that aren't reachable by the outside world. However, such addresses bear further investigation if you encounter them.[/b]
Good note about the non-routable IPs. You should still see the message enter a host through its Public IP and exit their private network with a Public IP as well. Usually it will be the exact same host or within the same Class B or C subnet.