I've had this bug before, the mail unread count for the email account inboxes is wrong, stuck at +1 for one of may email accounts. The badge is not showing. I've had this before and changed the number of sync period to days or 1 month to fix it. Has;t worked this time on IOS9, nor has a reboot or switch off. Still stuck with showing 1 unread email in the account list. It's a live (hotmail) account, and on the web mail it shows the same emails, and none unread.