Hmmmm.... I created a subfolder "Junk-Email", then re-named it to "Junk Email", then it worked, at least for now.

The "Junk Email" folder disappears every time i start Thunderbird, so I have to apply the re-naming hack to make it come back. A little pane comes up saying

       Thunderbird
       The current command did not succeed.  The mail server with the account
        ... mail responded: Target folder already exists.

I don't see this message in the Error Console anywhere, which is frustrating.

Also, I always sort my mail-messages with the newest at the top. When I re-name the "Junk-Email" folder, and the restored "Junk Email" folder reloads the messages, it still comes up with the default ordering of the newest at the bottom. I'm not complaining about the ordering, just noting that my setting for the folder doesn't persist, its like all the information about it gets erased every time. Do any of you know what's happening here?

The server locations of the various folders are

imap://<myname>%40XXX.com@outlook.office365.com/INBOX imap://<myname>%40XXX.com@outlook.office365.com/Junk-Email imap://<myname>%40XXX.com@outlook.office365.com/Junk%20Email

where "XXX" is my company name. So the mapping of folder names is consistent in the paths, though Thunderbird lists the first one as "Inbox" rather than "INBOX".

When I start up I do see the "Junk Email" folder listed originally, but I get messages like this

    Login to server outlook.office365.com with username <othername>@hotmail.com failed.

regarding other accounts, and the "Junk Email" folder disappears about the same time. I do know that Thunderbird does some authentication with the IMAP server, and likely it's doing it incorrectly when it maps the "Junk Email" folder or maybe it's just timing out.