2.1.9 [RFC5545] Section 3.8.4.3, ORGANIZER MUST Be Present on Group-Scheduled Calendar Entities

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The specification states, "This property MUST be specified in an iCalendar object that specifies a group-scheduled calendar entity."

Outlook does not export an ORGANIZER property on an exception to an existing recurring meeting in which the attendee list was not modified. For more details, see [RFC5545].