As Roady mentioned, use deferred delivery as an option set in Outlook. You
never mentioned what type of mail server that you use. If Exchange, the
deferred message is saved on the Exchange server and will be sent later
whether or not you have Outlook running. If not using Exchange, that
message remains in Outlook's message store awaiting the scheduled delivery
time, so if Outlook isn't running at that scheduled time then it cannot send
your delayed message.
If you have a problem remembering to issue follow-up e-mails then the
following add-ons might help: