Some users have noticed the problem of 1 (one) unread email in the Windows Mail App in Windows 10, even though there is no unread email in the account. In order to resolve this, do the following: a. Select the account in the Windows Mail App in which the problem of unread email is showing up. b. Select the first email in that account. c. Then in the right hand side from the top, choose, mark it as unread. d. Then again choose mark it as Read. It will resolve the issue. I don't know the logical reasoning of the same, but it works for me.