Deleting and re-adding your email account is sometimes the easiest way to resolve a problem with a specific Mail account. After deleting the account, perform a forced restart before re-adding the email account to your iPad.
Settings > Mail > Accounts > [Your Mail Account] > Delete Account
Force restart iPad - Apple Support
Settings > Mail > Add Account
Also be aware that some email providers now require the use of App Specific Passwords when accessing mail over IMAP or POP3. If this applies to your email account, you will need to generate a unique password for your iPad email account - that will differ from your normal Account Password. As examples - AOL, Yahoo, Google, Microsoft all use App Specific Passwords when using anything other than their own email client Apps.
If you need to generate an App Specific Password, you’ll need to log-in to your Mail Providers account management webpage - using a web browser - and generate the Password from the appropriate account security page.
More information about AOL’s App Specific Passwords can be found here:
https://help.aol.com/articles/allow-apps-that-use-less-secure-sign-in?guccounter=1
For Yahoo, more information can be found here:
https://help.yahoo.com/kb/SLN27791.html
For Gmail:
https://support.google.com/mail/answer/185833?hl=en-GB
Microsoft:
https://support.microsoft.com/en-us/account-billing/using-app-passwords-with-apps-that-don-t-support-two-step-verification-5896ed9b-4263-e681-128a-a6f2979a7944
