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

where "ABC" 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 initially and then it disappears.

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.

These are the path mappings of the folders:

    imap://cponder%40my-company.com@outlook.office365.com/INBOX
    imap://cponder%40my-company.com@outlook.office365.com/Junk%20Email
    imap://cponder%40my-company.com@outlook.office365.com/Junk-Email

So thunderbird re-establishes the path when I re-name the folder from "Junk-Email" to "Junk Email", though the re-naming doesn't happen, the above error pane shows up and the "Junk-Email" is preserved. What I don't get is why the path fails (and the folder disappears) when thunderbird starts up.

re :I tried re-adding it as a sub-folder, which is how I created it in the first place,

Certain folders, by default, are created by the server and those folders cannot be altered. They are not folders that you create nor rename. All default folders are not subfolders. Typically they are called Inbox, Drafts, Templates, Sent, Archives, Spam, Trash - although there are a few variants eg: 'Sent' might be called 'Sent Mail'

It would help if you could logon to the outlook.com webmail account via a browser and get a screenshot which shows all the various folders that are displayed on the server. This will help to show us what you see as default server folders and additional folders you have created.

I'm somewhat perplexed as to why you are creating a folder which should be, by default, a folder already created by the server. Whilst logged on to the webmail account - if you mark an email is spam which folder does the server automatically use?

According to this: https://support.microsoft.com/en-gb/office/working-with-message-folders-in-outlook-com-6bb0723a-f39f-4a8d-bb3f-fab5dcc2510a

Default folders By default, your account starts with these folders:

• Inbox
• Junk Email
• Drafts
• Sent Items
• Deleted Items
• Archive

If you have created other folders eg: 'Junk-Email' or 'Junk Items' etc then delete them. If you can delete them then they were not server default folders.

You are using an IMAP mail account so that means to see server folders you have to subscribe to see them:

• Right click on the mail account name folder in Folder Pane and select 'Subscribe'
• click on 'Refresh'
• Select folders
• Click on 'Subscribe'
• Click on 'OK'

You also could do with checking that you have the following settings b ecause they can effect which folders you have selected to see in the Folder Pane whether you have subscribed to see them or not - check you do not have any folder filters operating.

• View > Folders > 'All folders'
• View > Threads > 'All'

Once you have removed any 'junk/spam' folder you created and subscribed to see the correct junk folder on server, you then need to make sure your Account Settings are pointing to use the correct folder.

• In Account Settings
• Select 'Junk Settings' for the account
• At the bottom where it says 'Destination and Retention'
• Select 'Move new junk mesasges to'
• Select 'Other' and choose the correct server named folder which I suspect is 'Junk Email on account name'