This is under "Do not deliver before time"

Important! If your Outlook is closed,  or your computer is turned off or asleep at the scheduled time, the email will not be sent until the machine is back in use. 

It's important to have a good work-life balance so scheduling the sending of emails can be an effective way of respecting others boundaries, for example, delaying an email you wrote at 6pm to arrive at 9am or delaying it for a time that the recipient is more likely to have time to read it, like after lunch.

Delaying the Sending of Emails in Outlook