When a user exceeds usage limits, SharePoint Online throttles any further requests from that user account for a short period. All user actions are throttled while the throttle is in effect.
- For requests that a user performs directly in the browser, SharePoint Online redirects you to the throttling information page, and the requests fail.
- For all other requests, including CSOM or REST calls, SharePoint Online returns HTTP status code 429 ("Too many requests") or 503 ("Server Too Busy") and the requests will fail.
If the offending process continues to exceed usage limits, SharePoint Online might completely block the process; in this case, you won't see any successful requests and Microsoft will notify you of the block in the Office 365 Message Center.