A new delivery can be made based on a previous message by clicking the message row on the Reports page. The row turns gray. Then press Open the newsletter for re-delivery in the toolbar to open the message in the editor for resending. The new draft will have the same settings by default as the previous delivery, but you can edit this information when sending the message.
Recently sent messages can also be reopened for editing from the Compose message page, through the Sent messages tab.