The issue is not what we know, but what we don't. Keep in mind, all we have is what was leaked. We only have a tiny glimpse into the massive amounts of post-9/11 surveillance. We can ascertain that the government is doing extensive amounts of surveillance on large amounts of people, and that the NSA has either refused to tell Congress the scope of the programs or outright lied about it (take a look at this (http://www.wired.com/dangerroom/2012/06/nsa-spied/) article from a year ago , and look at Clapper's "Not Wittingly" statement).

To ask for "Links? Data? Do you have anything that supports this at all?" is asking the wrong question, since the amount that is public is incredibly small (just a tiny fraction would be my guess, but all we can do is make estimates). The question is, Is it reasonable to assume that there is more surveillance? The answer is yes, and to assume that there is detailed email surveillance, either at the ISP level, through backdoors, or through direct cooperation is not unreasonable.