How can I set up Mail so that it will send a message at a later time?
Apple’s Mail doesn’t include a scheduling feature, but you can add one. Andreas Amann’s free Mail Scripts (macworld.com/0446) includes a Schedule Delivery script. The idea is that you write your message and save it as a draft, then invoke the script from the AppleScript menu, which you can enable in AppleScript Editor. (You’ll find Schedule Delivery under the Mail Scripts heading in the AppleScript menu.)
When you invoke the script, you can choose messages in Mail’s Drafts folder and create a delivery date and time for each one. Once you do this, the script creates a Mail Scripts calendar in iCal. Additionally, an event is created that includes an alarm compelling the message to be sent at the appointed time.
Note that the script doesn’t appear to work with Microsoft Exchange accounts.