ezmlm-warn scans dir/bounce for bounce messages received by ezmlm-return. If it sees a distribution bounce for box@domain received more than ten days ago, it sends box@domain a list of all the message numbers missed recently, and deletes the bounce. If it sees a warning bounce for box@domain received more than ten days ago, it sends box@domain a probe, and deletes the bounce.
ezmlm-warn will not send a warning or probe to an address that is not currently a subscriber.