I think there's just a word gap. When sending old school paper mail you can say all of the following;

there's a lot of mail today

has the mail arrived

mail me the details

--BUT you would typically not say

i sent two mails

--INSTEAD you'd likely say

i sent two letters

When we replace mail with e-mail we have also replaced letter with e-mail. So when I say:

i sent two e-mails

Anyone trying to reverse engineer the obvious rule would see that as synonymous with;

i sent two mails

Thus because there's not a 1-to-1 relationship between the 'old' and the 'new' many who are applying the old grammar will not accept the new method, which when you fill in the gaps (ie with e-mail=letter) is perfectly fine