Outlook Connectivity with MAPI over HTTP | Microsoft Community Hub MAPI HTTP increases the CPU load in the Exchange Client Access Servers This is a 50% increase over Exchange 2013 RTM, however it is still lower than Exchange 2010 requirements As you plan be mindful that deploying in a multi-role configuration will minimize the impact to your sizing
Outlook Global Customer Service Support Team Blog Calendar sharing in Microsoft Outlook is a feature many users rely on daily to manage their manager or team calendars Microsoft has recently made some big changes to calendar sharing in Outlook and there are some key differences between the old calendar sharing model (which uses MAPI) and the new calendar sharing model (which uses REST) The intent of this blog is to inform IT Messaging
New Outlook: MAPI Exchange | Microsoft Community Hub I don't think microsoft will ever try to add mapi to it because mapi is a "client side" technology that whereas the new outlook is all server side (excluding the html css rendering)