[Dovecot] Courier to Dovecot: problems with folders

[Claudio Strizzolo]

Hi all,
I am migrating a maildir Courier-IMAP installation to Dovecot.
Almost everything seems to work well, but I still have problems 
accessing subfolders.
First let's see what I have done.
I used the courier-dovecot-migrate.pl script suggested in
http://wiki.dovecot.org/Migration/Courier to convert mailboxes from 
Courier to Dovecot. No problems here.
My dovecot.conf contains this:

(...)
mail_location = maildir:/mail/vmail/domains/%h
namespace private {
    prefix = INBOX.
    inbox = yes
}
(...)

The subscriptions file in the mailboxes looks correct to me. It includes
all the folders and subfolders, i.e.:

Sent
Trash
Drafts
mail.dovecot
mail.misc
mail.postmaster
tools
web.apache
web.css

Folders separated by "." are actually subfolders:

       mail ---- dovecot
             |
             --- misc
             |
             --- postmaster

I let the configuration of my IMAP clients unchanged from Courier to 
Dovecot. That is, they specify the "INBOX." prefix, as before. For 
instance, with Pine:

folder-collections=mymail {mymail.example.com/user=John.Smith}INBOX.[]

Now the problem: I get different behaviours depending on the client I 
use to access to Dovecot:

- Thunderbird:
   All the folders are seen correctly. But the whole folders tree is
   under "Inbox", while with Courier the first-level folders (i.e.
   Drafts, mail, web) were at the same level of Inbox

- Webmail (Squirrelmail 1.4.8):
   The same as above.

- Pine:
   The folders are apparently correct, and are not under Inbox, but I can
   access only the first-level folders: the folders containing
   sub-folders are empty, therefore the subfolders cannot be accessed.

Any hints?
Thanks in advance

Claudio