Why one Outlook client stops working when others are OK?
 

When outgoing mail from Outlook via mail.messagingengine.com on my wife’s desktop suddenly stopped working this week, my futile efforts to fix it elicited a message from Fastmail telling me I was using the wrong server name, and I learned that FM’s new system calls for app passwords and smtp.fastmail.com. My spouse was feeling very impatient because urgent email wasn’t going out, so I was feeling the heat, and took an quicker route of routing her outgoing mail via a different provider.

I rely heavily on Fastmail and use it all the time, so I'm not sure how I managed to remain ignorant for so long of this change in policy.

I have two other PC’s and a couple of mobile devices that are apparently still grandfathered in on the old server name and single password model, and they’re all working fine. That includes a desktop on which I had to reload Windows a couple of months back, which meant reconfiguring all my accounts in Outlook; I used mail.messagingengine.com, and all is still good. Why would one Outlook client (unfortunately, my wife's) stop working when others are fine?

While I cannot answer your questions in detail, the move towards the new server names and app passwords started years ago. Support for the old server name and the account master password (a bad idea for security reasons) is for convenience during a (long) transition period. I recommend that you change over to the new system as soon as convenient.