A company that we deal with has their internal network numbered with 18.104.22.168/24, specifically a mail server. That IP range belongs in China, causing all of their mail to be flagged as suspicious by our SPAM filter.
I found this rant about using other people's IP's on your network: http://mellowd.co.uk/ccie/?p=886
Do you know of any better articles/rants about this subject? This one is not convincing enough for me.