Issue Description: Although users are either correctly set up as members of shared mailboxes, or delegates with ‘Send As’ rights to user mailboxes in the Portal they still receive the following message in a non-delivery report when sending from the account in the Outlook Client, getting the error message as :
“This message could not be sent. Try sending the message again later, or contact your network administrator. You do not have the permission to send the message on behalf of the specified user.”
Latest Update and Guidance: This issues occurs when using an auto-complete entry that already resides in the Outlook Client auto-complete cache, and this needs to be removed and replaced with the ‘From’ mailbox selected directly from the Global Address List (GAL). To solve this issue, please follow the guidance available in the Microsoft KB article.