From Microsoft:

Customer Impact: A higher than average number of customers reported this issue. Analysis indicates that customers likely had some users experiencing this issue.

Incident Start Time: Wednesday, July 15, 2015, at 2:25 PM PST

Incident End Time: Thursday, July 16, 2015, at 6:30 PM PST

Preliminary Root Cause: A recent update to the Exchange Online environment caused excessive CPU utilization on the Active Directory service.

Next Steps: The following is a list of known action item(s) associated with this incident. As part of the Office 365 problem management process, additional engineering actions may be identified to improve the overall service. - Action: Review procedures for validating service updates to improve the change management process.

A post-incident report will be available on the Service Health Dashboard within five business days.

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