At Ignite, the Outlook team said that "coming soon" we'd have the ability (in an Insider version of Outlook) to manually enable the new calendar functionality via a checkbox (see below) to switch from MAPI to REST. Any update on when this will roll out to an insider build?
For Windows clients, there are two levels of Insider programs: Insider(formerly known as Insider Fast) and Monthly Channel (Targeted)(formerly known as Insider Slow). More info on the two levels here: https://insider.office.com/en-us/join/pc
We are planning for the new checkbox to be available for Insiders to try out early next year (January 2019).
Best Response confirmed by
Reed Wiedower (Occasional Contributor)
Great - we've got lots of folks eager to give this a spin. And just to clarify: does this checkbox actually move Outlook completely off of MAPI, or just the calendar functionality will move to a REST mechanism?
The checkbox will switch Outlook from MAPI --> REST for updating new-model shared calendars, specifically. This webpage describes what a new-model shared calendar is: http://aka.ms/calendarsharing.
Outlook will continue to use MAPI for updating calendars other than new-model shared calendars, as well as syncing down all calendars, and updating/syncing down all other item types.
(We do eventually plan to extend this and use the REST protocol for updating all O365 and Outlook.com calendars, either your own or ones shared with you, but we're starting with the calendars shared with you.)