[Dovecot] mdbox and folders containing both messages and sub-folders

[Rich Wales]

I've got Dovecot 2.2r5 (Ubuntu package version 2:2.2.0~rc5-0~auto+1
from the "testing-auto/dovecot-2.2" repository on xi.rename-it.nl)
running on a brand-new Ubuntu 12.04.2 server.

I'm trying to configure Dovecot (using the "mdbox" mailbox format) to
allow folders to contain both messages and sub-folders.  However, it's
not working consistently.

I need to have folders with both messages and sub-folders because I'm
trying to migrate mail from an existing (but soon-to-be-retired) Cyrus
server, and the existing body of mail has folders of this sort.

I'm currently using the following configuration line:

 mail_location = mdbox:~/mdbox:DIRNAME=.indexes:INDEX=~/mdbox/indexes

and am using the Larch program to do an IMAP-to-IMAP copy of one of my
family member's mail from the old Cyrus server to the new Dovecot server.
When I examine the "mailboxes" directory tree, I see some directories
that (correctly) contain both the renamed ".indexes" directory and also
directories for IMAP subfolders.  However, the Larch output is showing
many errors of the form "mailbox cannot contain messages", and some of
the messages could not be migrated because of this problem.

Am I doing something wrong?  Or is the "folders containing both messages
and sub-folders" feature simply broken for the "mdbox" format -- meaning
presumably that I'll be forced either to use the Maildir format, or else
abandon Dovecot entirely and install Cyrus on my new mail server instead?

Rich Wales
richw at richw.org