Microsoft is rolling out a feature update to prevent an issue where a meeting cannot be updated or canceled in Outlook because it has too many recipients.
Office 365 has a maximum of 500 recipients for any message. This affects calendar events because of the way that responses are currently recorded for meetings. For example, a meeting can be sent to a large distribution list (which only counts as 1 recipient). If responses are requested, each time an attendee responds to the meeting (accepting, tentatively accepting, or declining the meeting), the attendee is also individually added to the meeting recipients list.
If enough attendees respond, the number of recipients can surpass 500, and the meeting organizer is no longer able to update or cancel the meeting.
Microsoft is rolling out the first phase of improvements related to this issue. In this first phase, the goal is to ensure the organizer can always update/cancel the meeting. Therefore, once there are 400 recipients, no additional responses will be recorded. However, if the meeting is forwarded, those new attendees will still be added to the recipient’s table.
In a future update, they will ensure that responses will still be recorded, even after 400 responses. More details will be provided by Microsoft when those changes are released.
Microsoft will be gradually rolling this update starting this week, and they anticipate rollout completion by early May 2018.