As the Office 365 admin, you may have company requirements to allow some users to send email on behalf of another user. For example, you may want to enable an assistant to send email on behalf on their manager. When you allow the assistant to send email on behalf of the manager, the emails will say that they are sent on behalf of the manager.
Send email on behalf of another user
Go to Office 365 admin center > Users > Active Users.
Click or tap on the user you want, and then click or tap on mailbox permissions.
In the search box under Send email from this mailbox, type the name of the user or users that you want to allow to send email from this mailbox.
Once you've selected your users, click save.
Want to know how to send email on behalf of another user? Check out the following topics: