In this area you'll find a read-only version of your message.
To access this area, from the list of sent messages, click in the message you wish to edit:
Then scroll down to the message summary and click in the button More Details:
This will take you to a message composer wizard which is read-only and its sole purpose is for you to check the audience, content and options created for this message:
In this page you can also clone the message in order to quickly send a new message using the exact same details. This is done via the Options menu by clicking in the Clone button: