There are several ways to make the old Mail and Calendar app work (for now at least!)
https://answers.microsoft.com/en-us/outlook_com/forum/outlk_win-outtop_new-outsub_ofh/how-to-stop-the-new-outlook-from-automatically/34246983-2a13-4462-8a8a-df618c36ca3f?page=4
Some methods leave the old outlook installed. Other methods involve uninstalling it.
Here is another suggestion.
1- Click on settings and go to apps. Search for Outlook (new) and uninstall it.
2- Open windows store and search for Mail and Calendar app. Make sure it is installed. If not install it.
3- You can open it from Windows Store. Otherwise go to programmes and launch it from there. You can also go to the Windows programes and create a start menu icon or pin to the taskbar.
4- The Mail and calendar app should work as long as you NEVER close it. Instead of closing it, just hybernate your PC. If you ever close and reopen the Mail app or Shutdown and restart you PC then the new Outlook will take over again.
If that method is not practical then you can try this other method.
1- Click on settings and go to apps. Search for Outlook (new) and uninstall it.
2- Open windows store and search for Mail and Calendar app. Make sure it is installed. If not install it.
3- You can open it from Windows Store. Otherwise go to programmes and launch it from there. You can also go to the Windows programes and create a start menu icon or pin to the taskbar.
4- Go to https://github.com/matej137/OutlookRemover
5- Click on the green "Code" button and download the zip file.
6- Unzip the zip file and inside one of the folders you will find a file, "outlook.bat" Right click it and select "Run as administrator". This will install a dummy version of the new outlook.
7- Run the old Mail and Calendar app. Now it should work without the new outlook taking over. Check the "Readme" file for more information.
Please let us know if you get stuck and need further help.


Kind regards
Chris