In order to check the reason why messages are stuck in queue you can click directly on the dashboard here:
A list of full queue is shown.
By clicking on the green "eye" you can spot all issue about delivering.
The message field is the most significant; it will let you know why the queue is there.
Note: In this specific scenario, the system has finished all scanning operations and is no longer involved in message delivery.
What you see in the message field is the reply from the mail server.
Any problems are to be found in the recipient.