How to "activate" a mail directory?

Hello,

as I need a mailbox locally only just for filtering, I've tried to manually create it (using Maildir as backend):

1. Created a folder in the appropriate location under /home/vmail with subfolders cur, new, tmp.
2. Set the same user:group as for the other folders (i.e. vmail:vmail).
3. Restarted Dovecot and Email-Client (Thunderbird).

However, I cannot subscribe to the new folder. What am I missing here?

Kind regards, Peter