Sending, receiving, and organizing email in Outlook.com
I've been in the same situation as you including the fact that I am unable to update the OS on my 2010 MAC Pro so I'm stuck with OS 10.13.6. That is the problem. I spent a lot of time with Microsoft Customer Service. The end result is that I'm now having to use outlook.live.com for my multiple emails and MAC Mail is just sitting there. I also had deleted one of my .msn accounts and tried every suggestion here, e.g. using Exchange to try and add it and I even continue to try and add back with the original settings....as I said....now using outlook.live.com. Sorry for the bad news.