As you should be aware, Exchange Online has a 500 recipients limit per message (see https://technet.microsoft.com/en-au/library/exchange-online-limits.aspx#Receiving%20and%20sending%20limits).

When sending request meeting to a distribution list, the DL count as 1 recipient but may contains more than 500 members. If each member respond to this request meeting, they are added automatically as 1 attendee, which ultimately may reach the 500 recipients limit. This then blocks the organizer to edit/cancel the meeting because of this limit being reached.

An update is being deployed to allow meeting request to be edited or cancelled even if the 500 recipients limit is reached.

This update is going to be rolled out in 2 stages:

  • first stage: once 400 responses are received, no more responses will be recorded
  • second stage: all responses will be recorded. This second stage is not yet scheduled

The rollout of the first stage is planned to be completed by early May.

You need to communicate to your end-users about the new ‘temporary’ 400 recorded responses for meeting request.