How to create subfolders in Dovecot IMAP server if you use a MBOX mail store format.
Problem with MBOX is that "folders" of messages are in fact text files. A directories can not be embeded in the file . But this can be easily circumvented.
It really works. Just when creating folders in IMAP client (eg Thunderbird) enter at the end of the name "/" character. Dovecot then creates a folder (but not the text file, so messages can not be stored in it).
In these folders, it is not possible to combine messages with subfolders. These folders must contain a IMAP (sub)folder(s). But there is a way to overcome it. It is described in Dovecot wiki: http://wiki2.dovecot.org/MboxChildFolders.
I found it here: http://blog.workingweb.nl/2012/01/04/dovecot-with-mbox-imap-and-subfolders-or-target-mailbox-doesnt-allow-inferior-mailboxes/