It's been quite a long time since I have this little issue with the Email app. I don't know when, what Update or whatever caused this.
The mail app always shows me that there is one unread mail. Which is false, as far as I can tell. I select all mails via the pulley menu und mark all as read. Still the same.
This is not a big deal, but maybe somebody has had this issue before or knows a quick workaround. Otherwise, deleting and recreating should solve it, I guess. Just don't wanna delete my 1000s of mails now ;-P