I suggest checking account from your online signin and see how the folders are displayed there.

Thanks for the suggestion, David. In the online interface - same result EG subfolder Created in "Sent" appears as such. subfolder created in"Inbox" appears at same level as "Inbox" Web interface shows file properties of the two thus: subfolder in "Sent": parent folder is "Sent" subfolder created in "Inbox": parent folder is "---"

Have you verified with email provider that "INBOX." is the IMAP server directory? That may be a component of the situation. What happens when you create a subfolder of the inbox from the online account? How is it reflected in Thunderbird?

The server is managed via cPanel and I can not discern the file structure. I stumbled across "INBOX" or "INBOX." by experiment. If I set the account to TB defaults for advanced server settings all folders appear as children of Inbox. If I set the IMAP server directory to either of the above all folders appear to be peers of Inbox and subfolders can created in all folders except Inbox. The web interface (cPanel) shows the folders as a list, not as a nested tree. But if the folder is a subfolder it shows its parent. Inbox, drafts, sent, deleted etc. are peers with no parent. A subfolder of any folder except Inbox shows its parent. A subfolder of Inbox has "..." as parent. If I set up the account in another email client (MS Outlook) the folders appear as the nested tree I expect, with Inbox, drafts, sent etc. as peers and subfolders I create in Inbox nested in one level.

Remove the IMAP server directory as that explicitly tells Thunderbird that folder in the root of the account are to be found in the server directory "INBOX" (This is probably a case sensitive setting BTW)

You might also be seeing limits imposed by cpanel. IMAP is a networked thing and the server is considered canonical so if you do something in Thunderbird counter to server directives, it will be ignored.

Sorry, Matt, but I don't understand your post.

In the cPanel webmail interface (and in MS Outlook), drafts, sent,spam, deleted items and archive are all peers of inbox.

In TB, without the IMAP server entry, they all appear to be subfolders of inbox.

In an IMAP account I can create subfolders in all folders except the inbox. Subfolders created in the inbox become folders at the same level as the inbox. TB 140.2.0esr Screenshot of server settings, advanced attached. Thanks

Caused by the use of the inbox server directory.

Sorry, Matt, but I don't understand your post. In the cPanel webmail interface (and in MS Outlook), drafts, sent,spam, deleted items and archive are all peers of inbox. In TB, without the IMAP server entry, they all appear to be subfolders of inbox.

What is not to understand. You put in a setting that makes the sub folders of the inbox (including trash/Deleted and drafts etc) appear to be peers of the inbox and are here looking for support on how to change the fact the sub folders of your inbox appear as peers of the inbox. The purpose of that setting its to do exactly what you are saying you don't want done.

Webmail is a webpage how it manages the IMAP settings is nothing to do with IMAP, the same can be said of Outlook, which in it's heart is written as a client for Microsoft Exchange. Just folk use it as a general mail clients. Largely because Microsoft gave it away free 30 years ago. It was garbage then, but free trumps robust with most folks.

I think you will find the technical specifications for the IMAP list command here https://datatracker.ietf.org/doc/html/rfc9051#name-list-command

Thank you, Matt. I hadn't understood that deleted, drafts etc are normally subfolders of inbox. That hasn't been my experience previously. Until I changed mail server recently I saw those folders as peers of inbox, and was able to create subfolders in my inbox. That's with a TB account configured with defaults. Presumably I am getting different results now because my new mail provider has implemented IMAP differently. Thanks for the link to the IMAP speciification but it's way above my capacity to understand! I came looking for support because of my limited understanding and I'm grateful to anyone with more knowledge who is willing to share it.