Creating folders

Hi,

I managed to install dovecot 2.2.36 along postfix, with virtual users. When I send mails from my thunderbird client a Sent folder is created, which is mirrored on server by Maildir/cur/.Sent. If I try to create other directories the same thing happenes, adding to the created folder name . prefix. For example, created a_folder would be found on server under Maildir/cur/.b_dir. If I remove the client from thunderbird (with cached data) and reconfigure it, this folders are no longer available. How can I make them be available?

Indication for right documentation pages is much appreciated.

Thanks, Mura Andrei