The most peculiar thing is that links off of https://mail.google.com/ essentially mimics the copy & pasted links (i.e. null header.) However, https://plus.google.com, https://twitter.com, and https://facebook.com all present a referer header to the server.

There's probably room here to do some investigation as to what Google is doing to make outbound links from mail.google.com completely drop the referer. (Also, what about other web based mail clients? Yahoo? MSN? Aol? Corporate Outlook?)