Outlook has two options for deferring delivery - setting a rule to hold the mail for xx minutes and selecting a date and time for delivery in the options dialog of a message.

Using the rule to defer delivery is especially helpful for Exchange server users, as normally Outlook sends Exchange e-mail immediately. Anyone can use to it delay delivery of mail for a few minutes, either by setting no conditions and applying the rule to all messages or by delaying delivery for only those messages meeting certain criteria. To create a deferred delivery rule, open the Rules Wizard and select the option to start from a blank rule and choose to create an after sending rule. The deferred delivery option is near the bottom of the list. The maximum amount of time you can delay delivery is 120 minutes. (This option is available in Outlook 2002/2003, I'm not sure about earlier versions.)

Setting a specific time for delivery means you could create a message tonight and set it to send at 9 am tomorrow. Unfortunately, this feature is best described as "YMMV", it doesn't work reliably and when it does, the message may show the sent time as the time you created it, not when Outlook sent it. It works best with Exchange server accounts, but it's worth testing it on your own system to see if it works with your configuration. Access the setting from the e-mail Options button on a message form.

(YMMV = your mileage may vary. You can look up this and other Internet terms at NetLingo)