Once a message has been unsent you can no longer see it in messenger. But with a third party app you can now see all the messages that are unsent. Take note these applications only work when notifications are turned on for messenger. Notisave, Filterbox, and Notifcation History Log are all apps that will let you see unsent messages. These apps save every notification on your phone so you do not miss them. Now you can see all the messages people unsend to you.