At some point since the release of iOS 7, the Mail app has periodically stopped clearing its badge when all emails have been read. It can be a
frustrating thing to deal with at times but luckily there is a way to solve this problem. A very simple way to fix this problem was also baked in during
the few past releases of iOS.
If you run into this situation you can solve it very simply by following these steps:
Open the mail app

Tap the Edit button and select "Mark as Unread"

Now tap the Edit button and select "Mark as Read"

Once you have followed these steps, the badge on the Mail app will disappear from your home screen!
frustrating thing to deal with at times but luckily there is a way to solve this problem. A very simple way to fix this problem was also baked in during
the few past releases of iOS.
If you run into this situation you can solve it very simply by following these steps:
Open the mail app

Tap the Edit button and select "Mark as Unread"

Now tap the Edit button and select "Mark as Read"

Once you have followed these steps, the badge on the Mail app will disappear from your home screen!