If you receive an authentication failed message in Mail on iOS
Your iPhone can’t verify your email username and password with your email provider, and you might need to enter your credentials again.
You might receive an authentication failure message if your phone cannot verify your username or password with your email provider. This can happen after a password change, an expired security token, or if two-factor authentication requires a special app-specific password. To authenticate your email account again:
In the Settings app, tap Apps.
Tap Mail.
Tap Mail Accounts.
Tap the account with the authentication failed message.
Toggle Mail off and on again.
Enter your email address and password again if needed.
If you still have issues with authentication, you may to restart your iPhone or to delete the email account and add it back.