Microsoft Exchange 2007 You cannot respond to a meeting without an organizer

Our office upgraded our server to Exchange 2007 (SP1). However, users are still using Ms Office Outlook 2003 (SP3) and have been encountering this error message when accepting or rejecting some meeting requests: “You cannot respond to a meeting without an organizer. You must add an Organizer field to the item.” In addition, we noticed that the users did have the organizer field filled out. Those users accepting or rejecting meetings through their iPhones do not encounter the error message.  Users are also unable to use the delegate function in accepting or rejecting meeting. There are times that show the meeting has already been accepted even when they had not previously accepted it.  There are no noticeable similarities in the meeting requests that encounter the error to pinpoint the source of the error. Can you please help us resolve this issue? Thanks!