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Symptoms
When you send a meeting request, you can put Distribution Lists (DLs) on the To line as required attendees. When the recipient accepts this meeting invitation, your original meeting item shows that recipient as an Optional attendee instead of Required.
Once we choose - Yes option - the organizer is not getting the notification and the attendee is not getting added as Optional. So further comunication about the meeting is lost. Issue observed in case of New Outlook in Mac - Accepted the invite but the organizer didn't get any notification and no optional attendee. So the updates are lost. In each case for which the recipient was 'automatically' being added as optional to the invite, they were able to change the following setting. They navigate to the 'Conversations' section of the Team's associated Sharepoint site. This brought them to an Outlook for Office 365 URL.
Cause
When Outlook receives a meeting acceptance, the sender of the acceptance is compared to the recipients in the meeting attendee list. If that recipient is not listed, it is assumed to be 'optional' and added to the list of attendees.
Status
This is by design.
To work around this issue in Outlook 2003 or later versions, expand the DL as soon as it is added to the meeting list. If the DL contains other DLs nested in it those will also have to be expanded.