Outlook 2000 problem

I have a user that has full mailbox rights over another user. They have the other users mailbox showing in their folder list. However, when they try to forward a message on behalf of this user (they so have send on behalf of permissions as well) the get the following error message pop-up
‘cannot save sent messages to a folder marked read-only. the message will be saved to the sent items folder. do you want to continue?’
They can continue and the message does send, but user would like it so that this error doesn’t come up at all.
Can anyone help?