Collapse the Ribbon to get more space on screen

To collapse (or minimize) the Ribbon, do one of the following:

• Double-click the label of the tab you are currently using.
• Press Ctrl+F1.
• In the Ribbon displays options list, choose Show Tabs:

Outlook for Microsoft 365 offers an unique view of the Ribbon: partially collapsed that expands by clicking the Minimize the Ribbon button:

The partially collapsed ribbon shows only the most popular buttons:

To access any hidden button, click the ellipsis (…) on the right side to open the drop-down list with a full list of buttons.

While the Ribbon is minimized, click any tab to turn it back on temporarily. It will automatically hide when finish using it:

To hide the ribbon completely, just choose Auto-hide Ribbon in the Ribbon displays options list:

It hides the Ribbon and shows three dots near the upper-right corner of the screen instead. Click the three dots button to redisplay the Ribbon temporarily. When you return to the email body, the Ribbon hides again:

To expand the Ribbon back to its normal size when you want to have it displayed again, do one of the following:

• Double-click the current tab again.
• Press Ctrl+F1.
• In the Ribbon displays options list, choose Show Tabs and Commands:
  

See also this tip in French: Comment réduire le ruban pour obtenir plus d'espace à l'écran dans Outlook.