The new version of Outlook Mac will "Use New Outlook" by default. You can switch back and forth between the old and new interfaces by choosing Outlook | New Outlook | Open New Outlook to turn it on. To turn it off, choose Outlook | New Outlook | Revert to switch back.  

There are some issues with the new Outlook that users should be aware of when trying the new Outlook.

Features not yet available in the new Outlook for Mac

There are several features that are available in the current version of Outlook for Mac for Microsoft 365 that aren't yet available in the new Outlook experience.

• Contact lists
• Export .ics file
• Export/Import of .olm and .pst files
• Notes & Tasks views not natively available

If you are going to use the new Outlook, just keep these known issues and missing functions in mind.

New Features:

Customize the Toolbar -- many features you are used to seeing on the Toolbar may not be visible.  However, you can customize the Toolbar to include the items you wish to see (e.g, categories, reply, forward, etc).  See New Mac Outlook Customizing the Toolbar for more information. 

Customize look and feel -- you can customize the colors, light and dark mode, spacing between text and size of text

My Day

See your daily Calendar agenda from a new task pane in the main window for Mail.

Suggested times

When creating events, get suggested meeting times based on your recipients’ availability.

Calendar Improvements

Respond to meeting invites directly from the Reading pane. You can choose to email a message to the organizer and respond with your RSVP.

Suggested times - when creating meetings you can get suggested meeting times based on your recipients’ availability.

Create meetings by selecting a time block in the calendar grid. Add meeting details, including attendees and locations, and view attendee availability directly in the mini-calendar.

 3-day view in Calendar allows you to see more meeting and event details when you have a full schedule

Snooze lets you to temporarily remove an email from your inbox and scheduling it to reappear later, when you're able to respond.

More information on the new version and known issues can be found here:  https://support.microsoft.com/en-us/office/the-new-outlook-for-mac-6283be54-e74d-434e-babb-b70cefc77439

For a comparison between the old and new Outlook for Mac, see New Outlook for Mac features