How to find unread emails after updating to iOS 7 on the iPhone: simply type "Unread" into the inbox search field for either all inboxes, or specific inboxes.

After the update apparently the mail app looks at your entire account; if like me you don't delete everything once it's read, you might have dozens of "unread" emails dating back years. I had 3 pesky emails in 2 accounts that had been marked unread, and after wrestling with it and not getting a good answer from the fount of Google knowledge, I finally just gave up and typed "unread" in search as a last resort. And hey, there they were, from 2012.