Exchange 2007

Hi we are running a exchange 2007 server.  On the exchange we have 3 different doamin names. Main one, say onepence. I set up an extra domain caled onepenceproperty. Ive set up a email called onepenceperoperty can recivie emails but when reply get this message
You are not allowed to send this message because you are trying to send on behalf of another sender 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.



Have full permission this email but still get this everytime i reply