Thunderbird's help forums proved very helpful however: The older Win7 machine here still seemed absolutely broken, no amount of trying seemed to work.
which would explain the erratic behaviour, at least. How odd is all of that? I'm thinking this probably could be related to a SSL security certificate issue in the Microsoft mail servers, where some servers have expired certs and others don't. Then it seemed to have stopped working again completely. Sometimes on entering the password anew, it would start working again. > 'Login to server with username failed.' This morning the Win 10 version was intermittently working, then it would sudden go back to showing the error message learning Linux, as I am now desparate to get away from MS & Windows). (I know, but it was the way our ISP wanted to provide our email service and I didn't have time to argue the toss or learn how to run a mail server as well as everything else e.g. It seemed fairly certain it was a problem with authentication on the mail server side which is provided by Microsoft as part of an Office 365 sub. It affected two different Thuderbird installations, one PC was running Win 7, another was a Win10 laptop at a different location and up to date. I had this problem, it started unexpectedly at around 18:00 UTC yesterday (6th July 2022).
