After configuring a 'send as' or 'send on behalf of' user may receive this message:
"You are not allowed to send this message because you are trying to send on behalf of another user without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission." or "You do not have permission to send on behalf of the specified user."
You must check the following:
1- Ensure that you have configured correctly the 'send as' or the 'send on behalf of'
-To configure the 'send as' by Poweshell
Add-ADPermission "John Simpson" -User "Domain\User" -Extendedrights "Send As"
-To configure the 'send on behalf of' by Powershell
Set-Mailbox UserMailbox -GrantSendOn BehalfTo UserWhoSend
2- Ensure that the mailbox is not Hidden from address lists
- Open the EMC
- Open the concerned Mailbox
- uncheck the option 'Hide from Exchange address lists'