The outbox queue shows all the letters that currently are pending. In other words - all the letters that were sent to the recipient, but for some reason haven't reached the mailbox yet. When the letter reaches the recipient, this letter dissapears from the "Outbox queue".
To view the Outbox queue:
To delete a mesage from the queue, tap on the message and hold until You see notification that message is deleted. After deletion, this message won't be delivered to the recipient and won't be saved in the “Sent” folder.
If you were unable to find answer for your question, please write to firstname.lastname@example.org