I'll guess that you're using your iPad and the default email app which comes with it. (If not, please provide details of the device you're using and whether you're using a dedicated mail app or web-based access).
The simplest way to solve such problems is usually to delete the email account (
https://www.pickaweb.co.uk/kb/delete-email-account-ipad/) and then to create it again. Don't worry, you WON'T lose any existing emails; you're only deleting the account SETTINGS, not actual emails.