The other idea I hesitate to recommend. Since deleting the account did not change the mail storage size you could have a corrupted system and IF so the way to remedy would be a system restore. Takes a bit of effort and may not resolve the issue, hard to tell.
Just to be sure and cover the bases first try a Forced Restart. Hold down the Home and Wake/Sleep buttons at the same time for about 15 seconds until the Apple logo appears. You will not lose anything but a minute.
If a forced restart does not help try a System Restore ???
First backup your device using iTunes. Also import your photos to your computer and copy all important data. Try restoring from backup first and testing it. If it does not help, you may have to restore as New and reconfigure from scratch as the backup could be corrupted. For this reason it is important to have your photos and data saved separately from the backup. Here are the steps for a Restore:
https://support.apple.com/en-us/HT201252