Calendar update meeting problem

I have one user that has delegate rights to anothers calendar and can modify
all and delete all and send on behalf. She tries to update a reoccuring
meeting and set it for a different date just for this one occurance. She
sends the update to the other users to recieve this update and she get the
following error message. "You do not have the permission to send the message
on Behalf of the specified User." She is using Outlook 2003 Client.

Any help is appreciated.

Scott