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With Focused Inbox, mail is routed to one of two tabs: Focused and Other.
Your most important emails should be routed to the Focused tab, with less important information going on the Other tab. This Outlook feature can be turned on or off, and the user has control over 'teaching' Outlook how to route the mail by moving it from one tab to the other.
Turn Focused Inbox on: Outlook 365
Step One: In Outlook 365, select the View tab
Step Two: Select Show Focused Inbox on the Ribbon
The Focused and Other tabs will appear at the top of your mailbox. You'll be informed about email flowing to Other, and you can switch between tabs any time to take a quick look.
Turn Focused Inbox on: Outlook on the Web
Step One: Open Outlook on the web (OWA)
Step Two: Click on the gear icon 
then select Display settings > Focused Inbox
Step Three: Under When email is received, select Sort messages into Focused and Other. The Focused and Other tabs will appear at the top of your mailbox. You’ll be informed about email flowing to Other, and you can switch between tabs any time to take a quick look.
Step Four: Click OK
Turn Focused Inbox on: Outlook 365 for Mac
Step One: Open Outlook 365 for Mac
Step Two: On the Organize tab, select Focused inbox
The Focused and Other tabs will appear at the top of your inbox. You’ll be informed about email flowing to Other, and you can switch between tabs any time to take a quick look.
Note: You can set the badge count—the count of unread mail—to include only the messages in the Focused list. See Outlook > Preferences > Notifications and Sounds > Badge count to adjust this setting.
Change how your messages get organized: Outlook 365 and Outlook on the Web
Step One: From your inbox, select the Focused or Other tab, and then right-click the message you want to move
Step Two: If moving from Focused to Other, choose Move to Other if you want only the selected message moved. Choose Always Move to Other if you want all future messages from the sender to be delivered to the Other tab.
If moving from Other to Focused, choose Move to Focused if you want only the selected message moved. Choose Always Move to Focused if you want all future messages from the sender to be delivered to the Focused tab.
Change how your messages get organized: Outlook 365 for Mac
Step One: From your inbox, select the Focused tab, and then select the message you want to move.
Step Two: Select Message > Move to Other if you want only the selected message moved. Choose Message > Always Move to Other if you want all future messages from the sender to be delivered to the Other tab.
Step Three: Select Message >Move to Focused if you want only the selected message moved. Choose Message > Always Move to Focused if you want all future messages from the sender to be delivered to the Focused tab.
Additional information, including a video, can be found here: