Have an interesting one here...
We have a user who is working remotely out in UAE, he connects to his email using Outlook whilst connected through a VPN connection.
We have given the user 'Send As' permission on another account so that he can send email from them.
This works perfectly fine through OWA, the user is able to send mail as the other. But when he tries the same thing through Outlook he is getting an error of permission even though we know he has the correct permission.
"You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk."