DNS messages pending submission - Contains delivery status notifications (DSN), also known as non-delivery reports that are ready to be delivered by Exchange. The Delete All Messages (no NDR) and Delete All Messages (NDR) functions are unavailable for this queue.

Messages queued for deferred delivery - Contains the messages marked by the client for deferred delivery or messages simply awaiting delivery at a different time.

Failed message retry - Contains messages that have been marked as retry due to a delivery failure. This queue also does not have the NDR functions mention in the DNS messages pending submission queue.