Planning considerations for REST-based (aka new model) calendar sharing in Outlook

This post has been republished via RSS; it originally appeared at: Microsoft Tech Community - Latest Blogs - .

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 Admins of the key differences when considering whether to stay with the move to the new REST model or roll back to the old MAPI model. 

Important The shared calendar improvements are automatically enabled for Current Channel and Monthly Enterprise Channel starting with version 2112, and for Semi-Annual Enterprise Channel starting with version 2202. To determine if you are using the new calendar sharing model, see the Microsoft Support articles Calendar sharing in Microsoft 365 and Outlook calendar sharing updates.

First, let's talk about the MAPI model. MAPI has been used for calendar sharing in Outlook for a while and has remained largely unchanged for approximately twenty years. 

Typically, these are the two most common configuration issues we see with the MAPI model:

• A single manager granting editor rights to multiple delegates (only one editing delegate is supported under the MAPI model) 
• Conflicting Permissions granted to the delegate for a given manager (example: a delegate was granted both delegate access from the Outlook client and Full Access mailbox permissions from Exchange to their manager's calendar)

These two common configuration issues are documented in the following articles:

• Best practices for organizations when using the Outlook Calendar (microsoft.com)
• Conflicting permission sets when working with shared or delegated folders in Outlook - Microsoft Tech Community

The new calendar sharing model based on REST designed to avoid MAPI issues related to conflicting permissions and multiple editing delegates. This is a plus for REST and may be a reason to use the new model. 

That said, there are also some things that users could do in the old calendar sharing model that they cannot (at least not yet) do in the new model, such as the following:

• Adding attachments when responding to a meeting invitation
• Saving drafts of meeting updates in order to send them out later

Information on other functional differences as well as best practices for REST-based calendar sharing can be found here:

• Calendar sharing in Microsoft 365
• Shared calendars updates in Outlook for Windows (microsoft.com)

Note Keeping high item counts below the 5k threshold specified in Performance issues for too many items or folders - Outlook | Microsoft Docs is a best practice shared by both MAPI and REST.

There are also some known issues with REST-based calendar sharing that you should be aware of. The Outlook team is currently investigating these issues and will update the below article when more information becomes available:

Known issues with Outlook Desktop Shared Calendar Improvements (microsoft.com)