Throttling changes coming to beta Outlook APIs and Outlook-related APIs in Microsoft Graph

For the latest information on throttling for Outlook-related resources in Microsoft Graph and the Outlook REST API, see Microsoft Graph throttling guidance.

We are deploying changes to the Outlook APIs both on the Microsoft Graph endpoint (https://graph.microsoft.com/beta) and the Outlook endpoint (https://outlook.office.com/api/beta) that change when API requests get throttled. These changes are only for Outlook-related resources, other APIs on the Microsoft Graph are not affected by this change.

We are deploying this change to the beta versions of the APIs with two goals in mind. First, we will be monitoring the impact of this change to see if the limits need adjusting. Secondly, and more importantly, we want to gather feedback from developers regarding the changes.

New limits

In addition to the existing Outlook-related limits, API requests will be limited by two additional limits: number of concurrent requests, and number of incoming bytes. Both limits are evaluated per-app, per-mailbox.

Note: These limits are the initial values and are subject to change.

Concurrent requests

~~Each application is limited to 4 concurrent requests per mailbox.~~

Incoming bytes

~~Each application is limited to uploading (POST, PUT, PATCH) 15 megabits every 30 seconds per mailbox.~~

Throttling response

~~Throttled API requests will continue to receive a 429 HTTP status code with a Retry-After header indicating the number of seconds to wait.~~

The Rate-Limit-* headers that the Outlook endpoint returned under the existing limits will no longer be returned. Microsoft Graph did not return these headers, so this change does not affect Microsoft Graph.