Using the "Send Later" command
The Send Later command will enable you to compose messages and then send them at a later point in time.
When you're finished composing a message in the Compose window, go to File > Send Later. This will move the message to the Unsent folder, located in your Local Folders account. You can save as many messages as you like to the Unsent folder.
To send messages within the Unsent folder, select Send Unsent Messages from the File menu.