Yes. This is the problem even I have... Can't we use "two mails"? May be can't we use like "I received two mails or two emails from my friend who is in Chain"
"I received two letters in today's mail from my friend in…"
'mail' is a collective/mass/uncount noun, and refers to ALL letters and parcels that are in transit, going from ALL the people sending letters etc., to ALL the people receiving the letters and parcels; and also, to any item/s delivered to a house or business.
e.g.
"Mum, was there any mail for me today?"
'email' is used as an adjective - an email account with Yahoo - and as a noun, referring to the messages you receive via your email account.
'email' is countable, since it refers to the individual messages you receive, so "I got three emails from him, asking me if…"