You can view the sizes of the incoming and outgoing mail queues and view messages on the incoming mail queue if there is a problem with mail processing. You will also be able to view any messages that have failed processing, though this is normally unlikely unless there is a particularly badly formatted spam message that has somehow escaped rejection by the MTA.
Viewing queue sizes
Viewing incoming and failed incoming messages
First connect with MultiTool then choose the Mail menu option. You now have options to view the unprocessed incoming queue and the failed incoming queue.
For the unprocessed incoming queue you will be able to view an individual message.
For the failed incoming queue you will be able to view an individual message, update it, delete it or requeue it. Messages that failed processing will be automatically deleted after 14 days.