I had previously disabled the new outlook. Today, when I went to use the old Mail app I saw this message. 
Out of curiosity I clicked, "open (the new) Outlook". Well nothing happened and I can continue to use the old Mail app without any problems. I can still send and receive mail. The new Outlook can't launch because I disabled it using one of the several methods mentioned online. There is a small rotating circle in the top right of the old Mail app showing that Outlook is trying to launch but can't proceed. But that doesn't bother me. So every time Mail opens, I just have to click "Open Outlook" and the old Mail app continues to work.
Of course, later it may be the case that the old Mail app will really stop working. Then I will have no choice to switch to a new e mail client.
Just in case anyone is interested, it is possible to download previous versions of the old Mail and Calendar app here:
https://windows-mail-and-calendar.en.uptodown.com/windows/download/1046010608
Kind regards
Chris
PS The Outlook remover tool that has previously been mentioned works by installing a "dummy" version of the new Outlook that can't launch. So when Windows tries to install the real New Outlook it can't because it reads the registry and thinks it's already installed. The developers also provide a tool for reversing all the changes that have been made in case anyone wishes to start from scratch and install the real outlook from Microsoft. Please read the "Reasme" file on their site for a full explanation of how it works.
https://github.com/matej137/OutlookRemover