I had this problem a couple of weeks ago - when trying to add a new email account. The ghost emails appeared on a different account though. Very odd. Disappeared after a cold boot.
I am having this problem almost every couple hours. I find it happens when I cross over between wifi to cellular. I am on Exchange, and these mysterious messages appear, I can't read email, and then my iphone inbox has to rebuild itself for 30 days of email. It's very frustrating.
I have tried:
1. Rebuilding my iPhone
2. Shrinking my inbox by 70%. I don't think this should matter since my iPhone mail preference is only set to 30 days.