Windows Outlook Focused Inbox requires modern authentication

I make heavy use of Windows Outlook, especially its feature that allows you to have multiple Office 365 accounts set up in a single client. My ENow work account showed the Focused Inbox feature, but my personal robichaux.net account and the account from the free tenant Microsoft gives MVPs did not, and I couldn’t figure out why– the feature was turned on in the tenants, and I could see it working in Mac Outlook and Outlook for iOS.

Microsoft’s Rob Whaley pointed me in the right direction. It turns out that you must enable Modern Authentication for Exchange Online in order for this feature to work in Windows Outlook. I had it turned on for the ENow tenant, but not for the other two tenants. As soon as I turned it on with Set-OrganizationConfig -OAuth2ClientProfileEnabled $true, and relaunched Outlook, I saw the Focused tab pop up in those two accounts.

The technical reason behind this is only interesting to Exchange admins, but in case you were wondering: the Outlook REST API that Windows Outlook uses to access the Focused Inbox requires the use of Modern Authentication. No modern auth, no REST API, and therefore no Focused tab.