"The user account which was used to submit this request does not have the right to send mail on behalf of the specified sending account."
"The access credentials that you have specified have insufficient delegate permissions to send the e-mail message. Contact your Microsoft Exchange administrator to grant the required Permissions."
To Connect Windows Powershell to the Exchange Online Service
To Grant Full Access Permissions
To Grant Send As Permissions
Article ID: 2708727 - Last Review: 5 Jul 2016 - Revision: 1