Many email providers now require use of App Specific Passwords. Where required, 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 (Outlook, Hotmail) 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