The "factory"-argument to the constructorof mailbox.Maildir is not 
used as it should be.

First, it's default is set to rfc822.Message instead of MaildirMessage 
and then, inside the module's code, MaildirMessage is hard-coded as a 
message constructor.

If I need a derived class with custom attributes, I cannot use it.

Patch please?

Attached.
I am not a python hacker (by now). :) 
I wonder why the Message-declarations must be before their usage 
_inside_ the Maildir-class but it seems like. So the changeset is 
rather huge but the code changes are somewhat minimal. Just because 
the Message-classes must be at the beginning. Perhaps they should be 
outsourced. :)

I don't really have experience with that module, so I don't know if 
that patch has side-effects on older programs using this module. But 
as compatibility has been broken down (a bit) with the current 
version, it should not really matter. :)

I think the required patch is much simpler; only get_message() needs to
be updated.  I've committed this change to trunk as rev60095, and to
release25-maint as rev. 60096.