Email accounts accessed through the Mail app are managed not from Mail, but from iOS. So to add or remove an account, you will need to go through the Settings app, not the Mail app.
Note: This guide is for Apple iOS 12
To remove an email account from your iPhone Mail app, follow this basic procedure:
Go to Settings
Select Passwords & Accounts
Note: In earlier versions of iOS, tap Mail or Mail, Contacts, Calendars, then open AccountsSelect the email account you want to remove
Select Delete Account
To confirm, tap Delete Account or, in some cases, tap Delete from My iPhone.
