Exchange and Outlook 2000

We are using MS Windows Server 2003 Standard SP1 with Exchange v 6.5. All users are using Outlook 2003 for email and meetings. All except for 1 or 2 users that are using Outlook 2000... this is the problem. When ever the Outlook 2000 user tries to accept an meeting request she receives an error. Is this a compatibility issue between outlook 2000 and outlook 2003(or Exchange 6.5)?