Choose Continue Composing Draft from the Compose menu, then choose the draft you would like to work on from the submenu.
The Compose window opens with the draft loaded.
If the Compose window is already open, you can load the draft by choosing its name from the Use Draft submenu of the Format menu.
Continue with your edits and send the message as usual.
After you have clicked Send, Mailer posts a message that asks if you want to delete that draft from the file system once the message is sent.