[Dovecot] Leftover mbox behavior from uw transition

[Adam Porter]

Currently running dovecot 0.99.12 on a RHEL 3 server.  This system used 
to use UW imap with mbox mailboxes, is now using dovecot with Maildir. 
(Actual user shell logins are not permitted.)  Performance is 
tremendously improved.  However, we still have users that have their 
client imap root set to the "mail" or "mail/" directory.  (Can't 
reach/change them all easily.)

As a result, people are still (inadvertently) creating/using mbox-style 
mailboxes (sent mail folders most often) on the dovecot server (dovecot 
of course handles them without complaining) which grow and hurt server 
performance.

Now that we've figured this out, I think we could use mb2md in a script 
to convert these big mbox files to Maildir.  (Is there a better way?)

But what would be the smart way to prevent people from creating these 
mbox files and still have their client work correctly with their imap 
root set to "mail"?

We currently use:

default_mail_env = maildir:~/Maildir

Thanks!
-Adam