Sending emails on behalf of can be set up by following the steps below.
Enter the User Profiles Program, click on the required user and then click Update.
Click on the Email/ Printing Tab.
Navigate to the Email General Options area in the bottom left of the screen.
Tick the Allow Send On Behalf of box, if not already ticked.
Enter the required emails address in the Addresses box, if multiple addresses, separate them with a comma.
Click SAVE and EXIT.
Please note, sending on behalf of other email addresses still sends from your primary email account, on behalf of the other account. This means that signatures etc will not necessarily carry over. You need to also ensure adequate permissions are set up within Outlook for these accounts as well.
Please see the following article for reference:
Send email on behalf of someone else - Microsoft Support
If you or your System Administrator does not have access to User Profiles, please raise a new case online and reference the title of this article.