This is supposed to be done by `mailman/handlers/after_delivery.py`, but that handler sets `mlist.last_post_time` which is the Mailman 2.1 name of the attribute which is now named `last_post_at` in the mailing list model. The test in `mailman/handlers/docs/after-delivery.rst` doesn't catch this because it too references `last_post_time` which is set as a non-persistent attribute of the test list.