Office 365, Email, Exchange, Windows, How to grant someone permission to send messages on your behalf
How do I give someone the ability to send messages for me with Exchange and Outlook?
To grant someone permission to send emails on your behalf using Exchange and Outlook, follow these instructions:
Note: If you are using OWA, assigning delegate permissions is not a feature in OWA since you can't open shared calendars as you can in Outlook.
Outlook 365, 2016, 2013