I am not aware of all the available graphical mail clients out there,
but I know that mutt (http://www.mutt.org/) can recognize Maildir and
mbox (and a few others).
a friend of man has mounted a qmail server , created an account for me
and send me a message i reveived by mozilla
so , why can i receive my message in my own server ?
Or you could setup a .qmail file in the user's vpopmail directory (eg,
/var/vpopmail/domains/linux-pour-lesnuls.com/deny/) to put the mail
elsewhere - so that would be done on a per-user basis.
I think your best bet would be to just IMAP or POP the messages, even if
only locally. That way you can use Mozilla (or any other mail client)
to read the mail. (of course, that requires you to setup an IMAP and/or
yes like squirrelmail for example ?