Hi Naomi4,
Have you tried reinstalling the app and ensuring all accounts are removed? You can add it back using the steps here: Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch Specifically, the section quoted below.
"Restore a built-in app that you removed
- On your iOS device, go to the App Store.
- Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
- Tap
to restore the app. - Wait for the app to restore, then open it from your Home screen."
Once restored, as the screenshot shows some documents and data remained before deleting, ensure the mail accounts are removed and iCloud mail is turned off with the following steps.
"Remove email accounts
- Go to Settings
> Mail > Accounts. - Tap the email account you want to remove, then do one of the following:
- If you’re removing an iCloud email account: Tap iCloud, tap iCloud Mail, then turn off iCloud Mail.
- If you’re removing another email account: Turn off Mail.
- Note: To remove the email account from all apps on your iPhone, tap Delete Account."
You can find those steps here: Add and remove email accounts on iPhone - Apple Support
After removing, test a normal restart of your iPhone and check the storage.
You can also find helpful steps and recommendations to make more storage available here: How to check the storage on your iPhone
Best.
