Microsoft first acknowledged users in the Americas were unable to access Exchange Online mailboxes at 17:54 GMT on Thursday, and shortly after warned the disruption was possibly affecting Microsoft 365 globally.
While it launched an investigation and engineers began working to restore services, by 04:36 on Friday Microsoft was reporting only a “partial recovery” in some parts of the world.
By 13:32, the tech giant said there were “broader improvements” in North America, “sustained improvements” across much of Europe and “encouraging recovery signals” in the Asia Pacific region.
Microsoft said it restored services gradually due to the scale of the problem and to help ensure services remain stable once they are up and running again.
“We are deeply sorry for the disruption this incident has caused our customers and partners,” a Microsoft spokesperson said.
“We know the trust they place in our services and take that responsibility extremely seriously.”
Microsoft said it has no “ETA” for when services will be fully recovered.
A full post-incident report will be released once Microsoft has completed a review.
Experts said while workers may have been happy to not have email access for the day, the incident showed the dependence many organisations have on these cloud-based services.
“The global Microsoft outage is a stark reminder of how heavily we rely on a handful of big tech providers,” said Rob Demain, chief executive of cybersecurity firm e2e-assure.
Matt Saunders, field chief technology officer at tech consultancy Adaptavist, said it would be challenging to mitigate against a similar problem.
“Classic IT disaster planning strategies suggest using a second service for resilience, but in practice, it’s almost impossible for a business to function without tools such as Microsoft,” he said.
“In the wake of this outage, organisations must have a plan in place for how to work without such tools, for when incidents like this occur.”