To make it difficult, the setting is hidden under the users personal settings under the Email tab. For the user that is the From this setting needs to be set on
| Allow other Microsoft Dynamics 365 users to send email on your behalf. |
|
If you are not the user yourself, you can use XRMToolBox to set it. (and the Mailbox for the From user needs to be approved etc to send emails)
For User that is sending on behalf of the From, their security role Privileges needs to include Send on Behalf (down the bottom under Business Management Tab).
(to be honest its easier to configure a Queue to send outbound emails and use it)