"We will be retiring Outlook REST v1.0 API’s from Office 365, beginning November 2019 as communicated previously in MC124063 and MC143425. Over the last few years, we have been investing in services that help developers access information in Office 365 in a simple and intuitive way. As we make progress in this journey, some technologies no longer provide the best way to interact with Office 365 data. The API’s we are removing, have been superseded by the Microsoft Graph and Outlook REST v2.0 API’s.
You are receiving this message because our reporting indicates one or more users in your organization are using applications built using Outlook REST API v1.0."