Microsoft Rolls Out the New Calendar Sharing Model (opens in new tab)

An automatic upgrade is currently ongoing to move Outlook calendar sharing from the old MAPI-based model to a new REST-based model. The new model has been available to Outlook classic clients on an optional basis and is now becoming the default. The REST model is simpler and exploits the Exchange Online service instead of depending on client transactions. It’s also part of the transition to the new Outlook.