[Dovecot] Problem with Thunderbird and mailbox with SPECIAL-USE flag

[Sylvain]

Hi !

I have an old Courier IMAP and in front of it, I have put a proxy cache
with the great Dovecot.

SOGo webmail is connected to the proxy and works great (very very more
fast) but I have a problem with Thunderbird : mailbox with « SPECIAL USE »
aren't automatically detected.
To make that works, I must :
- in Dovecot : define the namespace inbox : « prefix = » and « separator =
. »
- in Thunderbird : force in the account, the personal namespace to "INBOX."
and untick « Allow server to exceed namespace » (sorry : personal
translation :D).

If I define in the namespace inbox as the dovecot wiki said : « prefix =
INBOX. » and « separator = . », TB see a sub-folder INBOX and mailbox with
special use aren't automatically detected. :-(
I must have forgotten a parameter in my Dovecot configuration but I don't
see it.. Can you have a look please ?
Here my configuration :

# 2.1.7: /etc/dovecot/dovecot.conf
# OS: Linux 3.2.0-4-amd64 x86_64 Debian 7.2
auth_debug_passwords = yes
auth_mechanisms = plain login
disable_plaintext_auth = no
first_valid_uid = 100
imapc_host = xxxxxx
mail_gid = imapproxy
mail_home = /var/local/cache/dovecot/%1Mu/%1.1Mu/%u
mail_location = imapc:~/imapc
mail_uid = imapproxy
namespace inbox {
  inbox = yes
  list = yes
  location =
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix =
  separator = .
  type = private
}
passdb {
  args = host=xxxxxx port=143
  default_fields = userdb_imapc_user=%u userdb_imapc_password=%w
  driver = imap
}
protocols = imap
ssl_cert = </etc/dovecot/dovecot.pem
ssl_key = </etc/dovecot/private/dovecot.pem
userdb {
  driver = prefetch
}

Sylvain