Consider the following scenario:
- You have Full Access and Send On Behalf Of permissions to a shared mailbox in an Exchange Server environment.
- You configure a Microsoft Outlook profile for the shared mailbox and enter your own credentials to access it.
- You send an email message from this Outlook profile.
And if you are running Outlook in Online mode, you receive the following error message:
You do not have the permission to send the message on behalf of the specified users.
In this configuration, Exchange Server requires Send As permissions to send the email message. If you don’t have Send As permissions for the shared mailbox, Outlook can’t send the message.
If user was typing in the “From” box, in that case always got an error. please select from the GAB.
Delete all email addresses from the “From:” box (which is seen when you send an email). The use the “display name” from the shared box in that field to re-add it. This will force that the client check the GAL and reload the correct permissions.
Ref: You do not have the permission to send the message on behalf of the – Microsoft Community