It might be worth noting that the email icon in TOHOLED appears only for any new emails. It disappears once you open the Email app. If you have 4 accounts and 10 new emails spread across all 4, opening just one of them clears the icon, even though you still have unread emails in the other accounts.

Also, I have just performed some experiments and have indeed observed that the icon does not appear for every new email. It definitely appears sometimes but I have to experiment a bit more to find the pattern.