I have a GW8 client who receives appointments from an external organization. He can accept them into his calendar, but he says the originator then receives back a "rejected" appointment.
GW 8 client and backend Sp2 HP3.
Don't know what external mail client originator is using (Suspect this might be pertenant ?).
I have iCal activated on my GWIA.
I also have "Enable Delivery Status Notification" set on the ESMTP TAB on the GWIA.