I’m sorry to be persistent.
When I searched Google, I got 9560 hits for “pieces of e-mail” and 5 hits for “articles of e-mail”. (As for snail mail, I got 80100 hits for “pieces of mail” and 630 hits for “articles of mail”)
So, can we consider that “piece” is commonly used as the quantifier for the mass noun “e-mail”?
If anyone knows a better quantifier, please let me know!