iCalendar to force the 'Accept/Decline' buttons to appear in an updated event instance for Outlook?

Does anyone know what iCalendar conditions I need to force the 'Accept/Decline' buttons to appear in an updated meeting message for Outlook 2007? Is there a combination of regular iCalendar fields, or some X-MICROSOFT fields I'm missing out on to force a re-accept? Our updates come far enough after the initial message to make a re-accept mandatory. I see this note for Outlook 2007, and I'm hoping that some combination of values will force an 'Accept/Decline' - short of a change of value in DTSTART. "If changes are made to meeting details, attendees receive an update with the changes highlighted, rather than having to reaccept the meeting." --http://office.microsoft.com/en-us/outlook/HA100743061033.aspx This problem occurs for updates to single, non-recurring, UID only meetings as well updates to recurring meetings using RECURRENCE-ID and UID. We use the ATTENDEE line in the update such as: ``` ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE; CN="Full Name":MAILTO:user@domain.com ``` to attempt to force the Accept/Decline behavior. Thank you everyone! ICAL messages are below. I needed to place the ATTENDEE records in the initial message to please Notes, but tested without those lines to the same effect. I also tried sending the initlal message as METHOD:PUBLISH and the updates as METHOD:REQUEST. The records update, but no new Accept button is offered in Outlook 2007. We bump the SEQUENCE number for each update. INITIAL MESSAGE ``` BEGIN:VCALENDAR METHOD:REQUEST PRODID:-//Product/Platform/Name//EN VERSION:2.0 BEGIN:VTIMEZONE TZID:America/New_York X-LIC-LOCATION:America/New_York BEGIN:DAYLIGHT TZOFFSETFROM:-0500 TZOFFSETTO:-0400 TZNAME:EDT DTSTART:19700308T020000 RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU END:DAYLIGHT BEGIN:STANDARD TZOFFSETFROM:-0400 TZOFFSETTO:-0500 TZNAME:EST DTSTART:19701101T020000 RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU END:STANDARD END:VTIMEZONE BEGIN:VEVENT UID:ical_event+5088@ical.server.org RRULE:FREQ=WEEKLY;INTERVAL=1;UNTIL=20091129T000000Z;BYDAY=SA DTSTART;TZID=America/New_York:20091114T180000 DTEND;TZID=America/New_York:20091114T190000 SUMMARY:Sub Test 7 ORGANIZER;CN="'Meeting'":MAILTO:ical_event+5088@ical.server.org DTSTAMP:20091027T212241Z STATUS:TENTATIVE SEQUENCE:5 CLASS:PUBLIC TRANSP:OPAQUE ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Full Name":MAILTO:user@domain.com ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Full Name":MAILTO:user2@domain.com END:VEVENT END:VCALENDAR ``` UPDATE MESSAGE ``` BEGIN:VCALENDAR METHOD:REQUEST PRODID:-//Product/Platform/Name//EN VERSION:2.0 BEGIN:VTIMEZONE TZID:America/New_York X-LIC-LOCATION:America/New_York BEGIN:DAYLIGHT TZOFFSETFROM:-0500 TZOFFSETTO:-0400 TZNAME:EDT DTSTART:19700308T020000 RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU END:DAYLIGHT BEGIN:STANDARD TZOFFSETFROM:-0400 TZOFFSETTO:-0500 TZNAME:EST DTSTART:19701101T020000 RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU END:STANDARD END:VTIMEZONE BEGIN:VEVENT UID:ical_event+5088@ical.server.org RECURRENCE-ID:20091114T230000Z DTSTART:20091114T230000Z DTEND:20091115T000000Z PRIORITY:5 SUMMARY;LANGUAGE=en-US:Sub Test 7 LOCATION;LANGUAGE=en-US:Access Code XXXXXXX DESCRIPTION;LANGUAGE=en-US:Sub Test 7 ORGANIZER;CN="'Meeting'":MAILTO:ical_event+5088@ical.server.org DTSTAMP:20091027T222122Z LAST-MODIFIED:20091027T221822Z STATUS:CONFIRMED SEQUENCE:7 URL:http://ical.server.org/calendar/detail.shtml?event _id=5088&instance_id=5385&recurrence_id=20091114T230000Z ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Full Name":MAILTO:user@domain.com ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Full Name":MAILTO:user2@domain.com CLASS:PUBLIC TRANSP:OPAQUE END:VEVENT END:VCALENDAR ```