Microsoft Outlook Shows email being sent “On Behalf Of”

The header is written like that to make it easier to track spam being sent from your server. You can do the following to stop it:

WHM > Service Configuration > Exim Configuration Editor > Options > and Un-Tick \”Always set the Sender: header when the sender is changed from the actual sender\”

Unchecking this will stop \”On behalf of\” data in Microsoft(R) Outlook, but may limit your ability to track abuse of the mail system.