How to delete data used by mail app on iPad?

I’ve deleted the mail app on my iPad however the data used by the app has not been deleted and still uses substantial storage. Prior to deleting the app I changed my password on my outlook account on outlook.com so the app deletion on the iPad would no longer be able to access outlook.com thus ensuring the mail app deletion on the iPad would not in any way effect my outlook.com emails.

Why has the data used by the Mail app not been released?

thanks!