Several users affected by the "throttling" error are reporting that service is restored, and Microsoft is stating the outage is over.
Posted 18 days ago. Sep 05, 2018 - 19:45 EDT
Some Outlook at Office 365 users are receiving a "throttled" error message when attempting to log in. This is due to a partial outage impacting Microsoft hosted services. Microsoft is reportedly making a change this afternoon to resolve this problem. We apologize for any inconvenience.