If the website is accessed via an HTTP Secure (HTTPS) connection and link points anywhere other than a secure location, then the referrer field is not sent.
The upcoming HTML5 standard will support the rel = "noreferrer" attribute / value to instruct the user agent not to send a referrer.
Source: http://en.wikipedia.org/wiki/HTTP_referrer
So it seems that referrer is not part of the domain, but inside SSL.
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