A delegate in Microsoft Office 365 invites a required attendee to a meeting on a principle’s calendar, which is located in a Microsoft Exchange Server 2010 on-premises mailbox. When the attendee accepts the meeting, the attendee is displayed as both an “Optional” and “Required” attendee.
This problem occurs because the calendar assistant in Exchange 2010 expects the principle and delegate to exist in the same forest (both on-premises or both in-cloud).
To resolve this issue, move the principle mailbox to Exchange Server 2013 or a later version of Exchange.
Article ID: 3182374 - Last Review: Jul 28, 2016 - Revision: 1
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