iPad mail not deleting

Are you talking about your e-mail account?:--> Settings>Passwords& accounts>[your account]>Delete account

or e-mail messages?:--> Open your Mail app, and delete messages, then empty your trash box

or Mail app?:--> Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

Regards

Giulio

What happens when you follow the steps provided in the first line of my reply?