You can delay the delivery of an individual message using Outlook webmail by following the below steps;

1. When drafting a new email, click the down arrow to the right of Send and click Send Later.

2. From here, you can select the time and date you need the email to be sent. 

The advantage of using the Webmail over the Outlook application is that your PC does not have to be left online for the email to be sent.  The disadvantage of using the Webmail is once you have hit send after setting the date and time, there is no way to change/stop the email.