Outlook Appointment arrives as mail

I have a situation where my Outlook XP user (POP3) receives something that I SENT to him as an appiontment yet it ARRIVES as another piece of e-mail so he is not allowed to accept the appointment and cannot save it as an appointment on his laptop. I send it from Outlook XP from a Windows XP workstation using an Exchange 2000 SP3 mail server. Any thoughts?
Marty