I emptied all mail including trash and junk and turned off the iCloud mail account on both devices. I do not use any .me or .cloud or .mac email addresses.
iCloud mail storage shows 19.88 MB is being used.
Logging out and back in doesn't free up the mail storage space. Restarting (completely shutting down) the iPhone 6 and iPad mini 3 still doesn't clear it up. Along came iOS9, which I've upgraded to, but iCloud storage still shows 19.88 MB used for mail storage. I decided to also log out and back in on my iMac and I checked the mail settings and the mail app and nothing is in there.
What I tried:
From the support page of https://support.apple.com/en-us/HT204247, I followed these instructions:
If your iOS device is configured to access iCloud email, you can delete messages from any mailbox, then empty the Trash to free up space:
Swipe left across any message to delete it.
Tap your Trash folder.
Delete messages one at a time or delete all of them by tapping Edit, then tapping Delete All.
