Qmail Queue...........

control/queuelifetime

When qmail queues a message for delivery, it will try mightily to deliver it --though not indefinitely. After messages have been in the queue for more than control/queuelifetime seconds, qmail will give up and bounce the message back to the sender as undeliverable.

The default queuelifetime is one week:

# bin/qmail-showctl | grep queuelifetime
queuelifetime: (Default.) Message lifetime in the queue is 604800 seconds.

I'm guessing this is longer than your users would like.

To bounce undeliverable messages more promptly, so senders will be alerted sooner about any delivery delays, set control/queuelifetime with a value equal to a day or so:

# echo "82800" > control/queuelifetime
# bin/qmail-showctl | grep queuelifetime
queuelifetime: Message lifetime in the queue is 82800 seconds.

Now the message will bounce if not delivered within 23 hours.