Has AT&T, your email provider, switched to using App Specific Passwords? If so, you will need to generate a unique password for your iPad email account - that will differ from your normal Account Password. Other examples are AOL, Yahoo and Google - all of which 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. You then use the App Specific Password when setting-up the account using the set-up wizard.
If you need to modify the account password after email account setup, you will need to update both the SMTP (send) and IMAP (receive) password setting.
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