Windows 10 Mail Indicates Phantom Unread Message

I am having a problem with Windows 10 Mail in which the icon in my task bar, and the icon in the start menu, indicates that there is one (1) unread message available. However, all messages have been retrieved and read.

I had this problem before, quite some time ago, and reported it here in another thread. At that time, the solution offered solved my problem. However, this time, the solution is not working. I can't seem to get rid of the phantom unread email count.

Well... after some period of time, it mysteriously cleared itself up.