The IMTA automatically returns undeliverable messages after a certain amount of time has elapsed. However, UUCP maintains its own queues for files, so it is possible for messages to get stuck in the UUCP queues where the IMTA's regular message return job cannot see them.
Thus, an additional periodic cron job is needed to return undeliverable UUCP messages. This job operates in the same way as the IMTA's regular message return job except that it scans the UUCP queues and not the IMTA queues. This job is scheduled by the cron daemon.