I had the same issue for the first time during this past month. Was driving me nuts that MAIL was showing an "unread" email, but I could not identify or locate it.
I did the REBUILD and the culprit went away . . . er, at least on my desktop computer. An "unread" indicator still shows up, however, when I open my MAIL on my IPAD.
Any ideas how also to clean up the MAIL APP on the iPAD?
This
article talks about iPhones, but it should apply to an iPad. Spoiler alert: the solution may not apply to your situation.
