This same thing just happened to me. My first sign of trouble was when I tapped on the mail icon, it crashed- would not open- crashed twice.
When I was able to get it to open without crashing, all of my emails that I had already downloaded and viewed were gone, and it went through and downloaded all of them again, even ones I had already deleted.
Also, the only mailbox I had was INBOX- the trash and sent folders were gone.
In those re-downloaded emails, I got several of the no sender emails, and in my case, there was a 2 next to it, as if it was a threaded email- even though it wasnt. Like everyone else, I could not open those emails nor delete them.
I rebooted the phone a couple of times, and they finally disappeared. I got my trash folder back after deleting some emails- right now everything appears to be back to normal
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