Microsoft is currently working on fixing the persistent “Unknown login activity” error that affects all Outlook users.
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It may be from the server.
The error in question is presumed to be caused by a configuration issue on the server side. So, there’s not much users can do to fix the problem.
Do not change your password
Normally, this notification should be sent when users log in from an unknown device in their accounts, while “Unknown sign-in activity” notification is constantly sent even though they are not logged in.
When users click on the relevant message, they are directed to the password change page, but changing the password does not work either; The error keeps coming. Even users who enable two-factor authentication encounter this warning message.
Microsoft says the bug is a configuration issue that is causing some users to receive these notifications by mistake. It is not known when Microsoft will release the update that fixes the bug.