I guess the only thing left is to submitt a support ticket with Apple. If you haven't already done so. As this is very odd.   CTS Support https://cts.net.au ... View more

Is the phone managed by your company? possibly a policy is blocking the delete action       CTS Support https://cts.net.au   ... View more

Go to Settings -> Mail -> Accounts -> Select the email account you want to delete by tapping on it  At the bottom of the page you'll see "Delete Account" ... View more