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Modern Authentication enables Microsoft/Office 365 advanced security sign-in features with Office client applications and it removes the need for Outlook to use the basic authentication protocol.
The Office client will use an in-application browser control to render the Azure AD sign-in experience in the same fashion as browser-based Office 365 clients like Outlook on the Web.
Modern Authentication allows the Office client to no longer need to store the credentials of the user on the client device.
Modern Authentication for Exchange Online only works with Outlook 2013 and later, supported web browsers, Outlook Mobile, Outlook for Mac 2016, and Exchange ActiveSync in iOS 11 or later.
If you use Outlook 2010/Outlook for Mac 2011 or earlier, basic authentication is still used because modern authentication is not supported on older versions of the Office applications.
Modern Authentication
Outlook 2016 Desktop client user experience
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Outlook 2019 desktop client user experience
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Outlook for Android user experience
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Outlook for iOS user experience
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