Focused Inbox separates your emails into Focused and Other tabs. If you would rather see all of your emails together in one inbox, you can turn it off.
Classic Outlook
Open Outlook and select the View tab.
Click Show Focused Inbox to turn the feature off.
The Focused and Other tabs will disappear and all messages will be shown together.
New Outlook
Open Outlook and click Settings.
Go to Mail > Layout.
Find the Focused Inbox section.
Select:
Don't sort my messages
Your messages will now appear together in a single inbox.
What This Change Does
Turning off Focused Inbox does not move, delete or change any emails.
It only stops Outlook separating messages between the Focused and Other tabs.