This article explains how to delete an email account on an iPhone. These instructions apply to iOS 12 and later, but the steps are similar for all recent versions of Apple's mobile operating system.

Email accounts accessed through the Mail app are not managed from within Mail, but from iOS. So adding or removing an account requires you to go through the Settings app, not the Mail app.

Scroll down and select Mail > Accounts.

Select the email account you want to delete.