[Dovecot] Can't remove folder

[Leroy van Logchem]

Hello,

Here is a 'me too' somewhat like:
http://www.dovecot.org/list/dovecot/2006-January/010759.html

Example scenario:
/u/username/Mail/subfolder/somebox
    subfolder is a directory
    somebox is a mbox

When I use Thunderbird's "Delete folder" function on the somebox the 
message appears:
"Target mailbox doesn't allow inferiour mailboxes".

Thunderbird, by default, tries to move it to Trash instead of deleting 
it directly.
When I change the preference to "Remove it immediately" it works.

Which component is at fault here:
  - limitation of a mbox only setup?
  - thunderbird should know
  - limitation of the imap protocol to advice the client it's mbox backed
  - feature of dovecot, could it be fixed or worked around somehow

--
Leroy