Page History

SpamAssassin, by default, will automatically attempt to figure out which Received: headers were inserted by mailservers in your network, and which were not. This auto detection works pretty well for most networks, but when Network Address Translation (NAT) is involved there are two possible configurations that SpamAssassin cannot automatically detect the difference between. One case is where your MX has a publicly routed IP, and the other where it does not. Looking at Received: headers alone, SA cannot distinguish between the two, so it assumes your MX has a public IP. Also, if you have a large or complex network topology, it's strongly recommended you set this (most ISPs, for instance).

...