Scheduling meetings with hundreds of attendees

Note: These changes apply only to meetings created in Microsoft 365that include hundreds of attendees. The changes don't affect smaller meetings.

In the past, when you used Microsoft 365 to send a meeting invitation to hundreds of attendees, you might have been unable to update or cancel the invitation. This occurred because Outlook listed each person who accepted, tentatively accepted, or declined on the To line of the invitation, and there's a limit to the number of individuals who can be listed on an email message or meeting invitation. If hundreds of people responded to the invitation, you would be locked out of your meeting.

To prevent this from happening, we’ve changed how Microsoft 365 handles responses to meeting invitations with hundreds of attendees. These changes ensure that you’ll always be able to update or cancel the meeting and still see the list of attendees. To view the attendee list for large meetings, you’ll need to use an updated version of Outlook. If you're using a version of Outlook that hasn't been updated, you won't see the full list of attendees.

What's changed in how responses are tracked?

How responses were previously tracked

In Microsoft 365, the maximum number of recipients on any sent item, whether it's a normal email message or meeting invitation, is 500. When you send an email message or a meeting invitation to a distribution list, the distribution list only counts as 1 recipient.

This was an issue for meetings because the list of meeting recipients and the list of responses were tracked by the same property on the meeting item. So the meeting organizer would send the invitation to a distribution list (1 recipient), and as members of the distribution list responded to the meeting, each person was individually added into the recipients list to track their response (eventually causing the recipients list to surpass 500 after enough people responded).

Outlook clients used the same property when displaying the list of recipients on the To line and when displaying the list of responses on the Tracking tab.

How responses are currently tracked

The list of recipients is still tracked by the legacy property, but the list of responses is now tracked by a new, separate property on the meeting item.

To ensure legacy clients continue to show individual responses for most meetings, Microsoft 365 will continue to track responses in the existing recipients property until there are 400 or more recipients. This means that there is no change in the experience for meetings with fewer than 400 recipients. After a meeting has 400 or more recipients, only updated clients will show the accurate responses for the meeting.

In Microsoft 365, the maximum number of recipients is 500, but a size of 400 was selected as the cut-off to stop tracking responses to give a buffer for the following two scenarios:

If a meeting is forwarded by an attendee, the forwarded recipients will always be added into the list of recipients, unless the list now includes more than 500 attendees.
If the organizer adds new attendees, the added attendees will always be added into the list of recipients, unless the list now includes more than 500 attendees.

Summary of changes

Meeting size	Legacy clients	Updated clients
400 or fewer recipients	All individual recipients and their responses	All individual recipients and their responses
Between 400 and 2,000 recipients	The first 400 recipients and their responses	All individual recipients and their responses
2,000 or more recipients	The first 400 recipients and their responses	The first 2,000 recipients and their responses

Notes:

If the organizer adds a new attendee to a meeting that already has more than 400 recipients, the new attendees will still be added.
We'll update Outlook in the future to show total counts per response type: Accepted, Tentatively Accepted, Declined, and Not Responded. This will help especially for meetings that have more than 2,000 recipients.
This article only applies to meetings organized by people using Microsoft 365 because the 500 recipient limit is specific to Microsoft 365. Administrators in Exchange on-premises environments can choose whether to have a maximum number of recipients and, if selected, what the maximum should be. For meetings organized by someone in an Exchange on-premises environment, clients continue to use the legacy recipients property for both the list of recipients and the list of responses.

Has your version of Outlook been updated?

The table below lists which versions of Outlook have been updated with the new method of tracking responses to a meeting invitation.

Client	Update status
Outlook 2016 for Windows	Not updated yet
Outlook 2016 for Mac	Not updated yet
Outlook on the web and Outlook.com	Not updated yet
Outlook for iOS	Not updated yet
Outlook for Android	Not updated yet
Windows 10 Calendar	Not updated yet

What if your version of Outlook hasn't been updated?

If your version of Outlook hasn't been updated yet, you'll only see up to up to 400 recipients and their responses. If you don't need to know who responded to your invitation, see I don't want to see responses to my meeting invitation.

I don't want to see responses to my meeting invitation

If you don't need to know who responded to your meeting invitation, you can turn off responses. This option is available in Outlook for Windows, Outlook for Mac, Outlook on the web, and Outlook.com.

When responses are turned off, you won't receive email when someone accepts, tentatively accepts, or declines the invitation, and responses won't be recorded in the meeting invitation. This is useful if you want to notify people about an event but don't need to know exactly how many people are attending, or if you don't want to receive email each time someone accepts the invitation.

Outlook 2016 for PC

A screenshot of the Request Responses button in Outlook 2016 for Windows