In Defaults.py, we have
DEFAULT_HOST_NAME   = 'localhost.localdomain'
MAILMAN_OWNER     = 'mailman-owner@%s' % DEFAULT_HOST_NAME

Then in mm_cfg.py, the user is expected to override DEFAULT_HOST_NAME with
DEFAULT_HOST_NAME = 'mailserver.example.com'
but MAILMAN_OWNER had already been interpolated with
'localhost.localdomain' -- meaning, for example, that when the
password reminders go out, they go out with

  If you have questions, problems, comments, etc, send them to
  mailman-owner.  Thanks!
                ^^^^^^^^^^^^^^^^^^^^^

even though DEFAULT_HOST_NAME has been set

Therefore, in the mm_cfg.py we ship, we should include both
a sample setting of DEFAULT_HOST_NAME, and then AFTER THAT LINE,
we should also copy in the
MAILMAN_OWNER     = 'mailman-owner@%s' % DEFAULT_HOST_NAME
line from Defaults.py so that MAILMAN_OWNER gets re-interpolated
with the newly, correctly-set DEFAULT_HOST_NAME

While it is possible for users to track this bug down and fix it
(I did...) it is a pain and the pain would be entirely removed
by this simple action.