I've been having this problem for probably about a year now and it drives me insane. I receive a ton of emails every day. I open each one individually (as opposed to swiping right on them) because otherwise, the Mail app won't register them as read. Even after this, the emails only stay 'read' for a few seconds before they mark themselves as 'unread' again. I will have to go through this process three or four times before it finally stays on 'read.' Occasionally, it also marks random emails from weeks prior as 'unread.' I've tried turning the phone on and off. I've tried deleting every email account and re-adding them. I've tried deleting the Mail app and redownloading it. I've done a factory reset. NOTHING is working. I'm about ready to ditch the Mail app altogether. Before I do that, does anyone else have this problem or found a solution?