Using classic Outlook for Windows in business environments
Hi, I'm Karl and will be happy to help you today.
Can you expand a little on where exactly you are getting this sign-in request?
Just to confirm something, when you state "my Outlook 365 Inbox" are you referring to the Outlook desktop program (part of Office) - the forum in which this question is posted?
Is this something that just started happening without anything else having been done in Outlook?
Is there a Yahoo based account (any email account using Yahoo servers) configured in Outlook '365. To verify, from within Outlook, go to
<File> --> <Account Settings> --> <Account Settings> --> <Email> tab
Once there is a clearer picture of your environment, will be able to provide a more specific response.