When you write an email in Outlook, you can 'schedule' the email to be sent at a later time instead of sending immediately.

To do this, write the email in the normal way. When you're ready to send the email, click the drop-down arrow next to the 'Send' button.

Next, click 'Schedule send'.

This will present some quick options for scheduling the email. If you want to enter a specific time, click 'Custom time'.

You can then choose the custom time, and click 'Send'. The email will not actually be sent until the time you specified.