Users who have these versions of Outlook for Mac will be unable to accept a proposed new meeting time without manually modifying the meeting request.
The Propose New Time feature is not available in Outlook for Mac 2011 and Outlook 2016 for Mac versions 15.8.2 and earlier. has declined, but proposed that you change the meeting time and then send an update. has tentatively accepted, but proposed that you change the meeting time and then send an update.
The New Time Proposed email message displays one of the following messages: In Outlook 2016 for Mac or Outlook for Mac 2011, you cannot accept a New Time Proposal from an attendee.
You cannot accept new meeting time proposals in Outlook for Mac