In Microsoft Outlook 2016 for Mac or Outlook for Mac 2011, you cannot accept a New Time Proposal from an attendee. The New Time Proposed email message displays one of the following messages:
<Attendee Name> has tentatively accepted, but proposed that you change the meeting time and then send an update. Proposed new time: <Date, time>
<Attendee Name> has declined, but proposed that you change the meeting time and then send an update. Proposed new time: <Date, time>
The Propose New Time feature is not available in Outlook for Mac 2011 and Outlook 2016 for Mac versions 15.8.2 and earlier. 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 available in Outlook 2016 for Mac version 15.9 and later versions when you are connected to Microsoft Exchange Server 2013 Service Pack 1 (SP1) or a later version. To obtain version 15.9 of Outlook 2016 for Mac, apply the April 14, 2015, update for Outlook for Mac for Office 365. To do this, see the following Microsoft Knowledge Base article:
3055707 MS15-033: Description of the update for Microsoft Outlook for Mac for Office 365: April 14, 2015
To use this feature when a new time proposal is received, open the New Time Proposed message, and then click the Accept Proposal or View All Proposals button.
Note If you are running Outlook 2016 for Mac version 15.9 or a later version and you are connected to a version of Exchange Server that's earlier than Exchange 2013 SP1, the Accept Proposal and View All Proposals buttons do not appear.
This workaround applies if either of the following conditions is true:
You are running Outlook for Mac 2011 or Outlook 2016 for Mac version 15.8.2 or an earlier version.
You are running Outlook 2016 for Mac version 15.9 or a later version and you are connected to a version of Exchange Server that's earlier than Exchange 2013 SP1.
To work around this issue, open the meeting from your calendar to modify the time, and then send the update to the attendees.
For more information about proposing a new time in Outlook for Mac, see the following Microsoft Knowledge Base article:
2935213 You cannot propose a new time for a meeting in Outlook for Mac