A message queue is a temporary holding area for messages before they reach their destination. The messages queues are: inbound, outbound, and delivery. The message queue size fluctuates based on mail flow.
If a queue continually grows without decreasing in size, there is a problem with message delivery. You can view the messages that are queued at any time to troubleshoot an issue. The message queue provides details about why the MTA could not deliver a message, how many times delivery was attempted, and when the next delivery attempt is scheduled.