Another bit of interesting information I found while doing some research for my dissertation, take a look at this oral history on the CHM site:
http://www.computerhistory.org/collections/catalog/102658003. Warner Sinback, in discussing his work with General Electric, talks about creating an email system. The timeframe is not clearly defined but it is obvious from the text that it is prior to 1979, easily placed in the early 1970s and perhaps as early as the late 1960s.
The body of 'prior art' accumulates. It certainly seems that the idea of email was a parallel evolution across the computer industry, not a point event - and certainly not in 1979 (or whatever date Ayyadurai is claiming this week). -- Ian