[forwarded from http://bugs.debian.org/384512]

the factory defines what class the mails have that are
returned.

So for two nested folders a and a.b, the following code
will return
messages with two different classes:

# factory = None to get mailbox.MaildirMessage objects
md = mailbox.Maildir("a", factory=None)
print md["somemessage"].__class__
# will print mailbox.MaildirMessage
md2 = md.get_folder("b")
print md2["someothermessage"].__class__
# will print rfc822.Message

i.e. the factory= parameter passed to the outer Maildir
class upon
creation is not passed on to the subfolder creation in
get_folder()