Microsoft Office 365
Microsoft customers in Europe, Asia and the U.S. were locked out of their email accounts on April 6.
The U.K. was hit especially hard by the Office 365 outage, with some businesses not able to send emails or access Skype for much of the day.
Some users reported that they could only log in to the popular office productivity suite using single sign-on.
The failure came a day after Microsoft introduced new security features to protect Office 365 users.