Mail box is stuck in inbox.
,
iPad, iOS 8.4.1
Try to reset your device. This won't delete your data stored on this device.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
Alternatively, you may go to Settings - General - Reset - Reset All Settings
https://support.apple.com/en-us/HT201559
If this does not help, restore your device to factory settings. Note that this will erase the data from your device.
Take a look this Apple Support article: Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.
Try setting up your mail account from scratch.
Add an email account on your iPhone, iPad, or iPod touch - Apple Support