Using New Outlook on Windows for personal email, calendar, and contact management
To switch from the new Outlook back to the classic version of Outlook, click the gear icon at the top right of the window. Then go to General. Go to About Outlook. From there you will have the option to go back to the old version of Mail. Even thought it says that you are going to the old version of the Mail app, it will also switch your Outlook back to the classic version. It will warn you that Windows Mail is being replaced with Outlook in the future, if you go to the old version of the Mail app, that will switch your Outlook back to the classic version.