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Hi. My name is***** you for your question and the opportunity to assist.
Do you have 2-step authentication set up on your Yahoo! account?
Another thing that could be causing this is that Yahoo considers Outlook to be a less secure app
and that can cause errors that can be corrected by making some changes to your Yahoo security settings.
Sign in to your Yahoo security settings here: https://login.yahoo.com/account/security
Turn on "Allow apps that use less secure sign in"
by moving the slider to the right such that it turns blue
and let me know if Outlook is giving the same message.
It may take a few minutes for the change to propagate through the Yahoo system
Thank you. I'll be working then as well.