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Have you tried going into Windows Credential Manager under the user profile and removing the stored Outlook credentials and then starting Outlook again?
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Owner

lloving

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OK. &nbsp;If I turn on Modern Authentication first and then turn Security Defaults on (in that order) Outlook on the desktop opens successfully using MFA. I find it hard to believe the order makes a difference, it may be simply "practice makes perfect". <br>My conclusion is this<br>1. Turn on Modern Authentication<br>2. Turn on Security Defaults<br>3. Clear Credential Manager<br>4. Reboot Desktop<br><br>Thank you Saif
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Welcome again and thanks for extra points.<br><br>
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I am trying to set up MFA on a small Microsoft 365 tenant. I have enabled Security Defaults Azure Active Directory Properties per the Microsoft document&nbsp;<h2>"Set up multi-factor authentication". Now the user cannot log into desktop application Outlook In Microsoft 365. Modern Authentication is turn on and legacy per user MFA is off. What am I missing?</h2><br>Thanks
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I am trying to set up MFA on a small Microsoft 365 tenant. I have enabled Security Defaults Azure Active Directory Properties per the Microsoft document 
"Set up multi-factor authentication". Now the user cannot log into desktop application Outlook In Microsoft 365. Modern Authentication is turn on and legacy per user MFA is off. What am I missing?
Thanks

Cannot log into Outlook after implementing Microsoft 365 MFA

Microsoft is best known for their Microsoft Windows line of operating systems, the Microsoft Office suite, and the Internet Explorer and Edge web browsers. Microsoft's mission is to "enable people and businesses throughout the world to realize their full potential by creating technology that transforms the way people work, play, and communicate.” Currently, the Windows operating system created by Microsoft powers over a billion devices around the world. On top of their share in the software creation market, Microsoft manufactures hardware, including laptops, phones, and attachments.

Microsoft

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

Office 365 is a group of software plus services subscriptions that provides productivity software and related services to its subscribers. Office 365 allows the use of Microsoft Office apps on Windows and OS X, provides storage space on Microsoft's cloud storage service OneDrive, and grants 60 Skype minutes per month. Office 365 includes e-mail and social networking services through hosted versions of Exchange Server, Skype for Business Server, SharePoint and Office Online, integration with Yammer, as well as access to the Office software. All of Office 365's components can be managed and configured through an online portal; users can be added manually, imported from a CSV file, or Office 365 can be set up for single sign-on with a local Active Directory using Active Directory Federation Services.