Apple - How can I send a mail message later?
It is possible to achieve this effect using Calendar and Automator:
Open Automator and create a Calendar Alarm.
Use the tasks “New mail message” and “Send Outgoing Messages”, customize them as required, and save (⌘S).
An event will be created in Calendar automatically. Move the event to when you want the message to be sent.
There is a plugin for Mail app which name is SendLater:
No promises, because some of the scripts break under OS X 10.7 Lion, but you might find your answer by using Andreas Amann's Mail Scripts which includes
Schedule Delivery (Mail)
Allows you to send individual messages at predefined times (this script uses iCal for scheduling message delivery).
Andreas makes the source code of his compiled scripts available. You can download the .dmg