I ran an ngrams query for the percentage of times that 'email' is used compared to the sum of 'email' and 'mail'. Not surprisingly, the graph showed a rapid increase beginning in the mid-1980s, reaching about 18% by 2008.
BUT - cue the spooky music from the X-Files here - from about 1810 to 1860 the graph was consistently in the range 6-10%, then only gradually faded up until about 1900.
Fox Mulder will be pleased to finally have proof !
Does anyone know what the word 'email' was being used for so frequently in the first half of the 19th century?