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You cant send a message on behalf of this user unless you have permission to do so

You configured 'Send as' permissions on the distribution group by going to Properties > Security and add the permissions on the distribution group.

A user who has 'Send As' permission on a distribution group couldn't send e-mail as the distribution group.  

The error message is :

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.

To resolve this issue

  1. Close Outlook
  2. Delete/Rename the offline address book folder under 'C:\Users\<username>\AppData\Local\Microsoft\Outlook\Offline Address Books'
  3. Open Outlook and download new OAB.