Outlook 2003 Send AS

When using the sending an Email from user A's box using from user B I get an error message from systems administrator

The following recipient could not be reached:[email protected] on dd/mm/yy hh:mm AM
You do not have permission to send to this recipient. For assistance contact your system administrator.
MSEXCH:MSExchangeIS:/DC=local/DC=domain:server


Any help would he appriciated.