"Authentication Failure for Mail on iPhone: When I go into mail, I am getting an error message. Authenticationfailure. Authenticate invalid credentials. I’m not aware of changing anything. It Worked yesterday. I’ve tried to find an answer on the internet with no success. Can someone please help me fix my email? Thank you."
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Fixing your Credentials:
A. Use a New App Password:
Note that you may need a new App Password, being that Mail is a third party app. You cannot log in using your eMail address password. As an example, you'd go here for gmail: Sign in Using App Passwords - Gmail Help - Google Support
No success?...
B. Add the Account Back:
I've seen quite a few relevant report on this since iOS18 and Sequoia, in regards to Outlook/Hotmail accounts in the Mail app. The solution for that is to remove the eMail account, and then add it back, using the exchange option.