Short glance into it, and after
commenting out 
    self.c_sock_close = self.c_sock_shutdown = True
in the parent process it ends up as
(maybe i can spend more time this evening):


15:36 ~/tmp $ python3 -E -Wd -m test -r -w -uall test_mailbox
Using random seed 9754656
[1/1] test_mailbox
/Users/steffen/usr/opt/py3k/lib/python3.3/mailbox.py:723:  ResourceWarning: unclosed <socket.socket object, fd=5, family=2, type=1, proto=0>
  return self._toc[key]
/Users/steffen/usr/opt/py3k/lib/python3.3/mailbox.py:77: ResourceWarning: unclosed <socket.socket object, fd=5, family=2, type=1, proto=0>
  return self.get_message(key)
1 test OK.
[118589 refs]