Calendar cancellation not reaching all attendees

[vandehey]

Hi! A user had a calendar event, that had 8 attendees on it. She canceled the event, and the cancellation only went to 4 of the 8 attendees. I looked in her sent items, and I see the calendar event notification. In the body of the message, it lists 8 invitees. But the headers of the message only has 4 people on the cancellation message.

I can't figure out how this happened. I did notice one thing though. There were external invitees on her list. 2 hotmail accounts. Could that cause this? Any other causes you can think of?

Thanks for the help