Situation: The userA has full permissions and send as permissions of another userB. However, when userA tries to send email as UserB, userA receives undeliverable message: This message could not be sent. You do not have the permission to send the message on behalf of the specified user. Error is [0x80070005-0x000004dc-0x00000524].
1. With Outlook open, go to Send/Receive>Send/Receive Group, download Address Group.
2. Repair Office.
3. Reset Outlook profile.
4. Deleting everything within the “Offline Address Books” folder. To do that, close Microsoft Outlook and try deleting everythingwithin the “Offline Address Books” folder. It will re-populate.
In Windows 8 it is located here: C:\Users\Username\AppData\Local\Microsoft\Outlook.
5. Uncheck the Use Cached Exchange mode option in the Outlook 2016 client for a test.