"$this" can no longer be used in a plain function or method since PHP 7.1.

The module currently failed PHP7.2 test due to the $this in this .module file:

   if (!empty($mail)) {
      /* @var \Drupal\message_notify\MessageNotifier $message_notifier */
      $message_notifier = $this->container->get('message_notify.sender');
      // @todo - figure out how to pass the email addresses to the notify plugin / hook.
      $message_notifier->send($message, array('mail' => implode(',', $mail), 'email'));
    }

I think this can be fixed simply changing to call \Drupal::service('message_notify.sender') instead. Submitting a patch for this.