Outlook includes a feature called Focused Inbox which intelligently organizes emails into Focused or Other based on your interaction. The more you use it, the better it gets at deciding which emails are most important to you.
However, you can always turn off the Focused Inbox if you prefer to see all of your inbox in one tab.
For the Outlook web app:
- Click the Settings gear, then use the search box to find Focused Inbox.
- Select Don't sort messages.
- Click Save.
For the Outlook mobile app:
- Open the Outlook mobile app.
- Select Settings at the bottom of the screen.
- Toggle the Focused Inbox switch to the off position.
Check out Microsoft's support site to learn more about Focused Inbox.