Attendee responding to a meeting invite and to all other attendees.

Hi,

We have a strange problem with one of our meetings, yesterday (october 09) an attendee of a meeting held on september 27 sent his invite message back to all the other attendees of that meeting, we don't know how this attendee got the e-mails of all the other attendees, the field "To" on his message had the e-mail adress of all the other anttendees. Does anyone have any idea what might have happened?

Thanks.