The default mailbox factory is mailbox.mboxMessage so I expect the following two statements to work the same:

messages = mailbox.mbox("test.mbox")
messages = mailbox.mbox("test.mbox", mailbox.mboxMessage)

However they do not.

The attached file generates the output:

<class 'mailbox.mboxMessage'>
Test
sender@invalid Thu Nov 17 00:49:30 2016
None
<class 'mailbox.mboxMessage'>
Test
MAILER-DAEMON Thu Sep 21 01:31:15 2017
None

Note that the original from has been lost in the second parse.

To me the documentation doesn't quite look right, in the case of no factory being passed, it runs:

    def __getitem__(self, key):
        """Return the keyed message; raise KeyError if it doesn't exist."""
        if not self._factory:
            return self.get_message(key)
        else:
            return self._factory(self.get_file(key))

from the Mailbox base class. The get_message is thus:

    def get_message(self, key):
        """Return a Message representation or raise a KeyError."""
        start, stop = self._lookup(key)
        self._file.seek(start)
        from_line = self._file.readline().replace(linesep, b'')
        string = self._file.read(stop - self._file.tell())
        msg = self._message_factory(string.replace(linesep, b'\n'))
        msg.set_from(from_line[5:].decode('ascii'))
        return msg

where self._message_factory is set to mailbox.mboxMessage.

I am not familiar with this module, so am unsure whether the documentation is ill-worded or it is a bug.