2.1.7 [RFC5545] Section 3.8.4.1, ATTENDEE MUST NOT Be Present When Publishing Calendar Information

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The specification states, "This property MUST NOT be specified in an iCalendar object when publishing the calendar information (e.g., NOT in an iCalendar object that specifies the publication of a calendar user's busy time, event, to-do or journal)."

Outlook can export ATTENDEE properties when exporting a calendar in the iCalendar format.