Microsoft said its servers were beginning to return to service after a global outage left users struggling to access its 365 suite of software, including Outlook and Teams.
Outage tracking website Down Detector recorded a sharp spike in problems, with nearly 130,000 reports of issues affecting access to Word, Excel, PowerPoint, Outlook and Teams, as well as enterprise services such as SharePoint and OneDrive. The company's AI assistant, Copilot, was also hit.
Although new reports later fell from that peak, many users said they were unable to send or receive emails, while others had trouble loading their inboxes at all.
Posting on X, Microsoft said it was "investigating reports of users experiencing issues accessing Exchange Online mailboxes through multiple connection methods". The company later explained that a portion of its infrastructure was "experiencing elevated load", which it said was "likely contributing to the connectivity errors". It determined that a number of servers were "unexpectedly unavailable" because of a problem that caused them to fail during automated checks.
As of 9pm UTC Thursday, the tech company said it had validated an alternative health check method on a subset of the affected servers, allowing them to successfully return to service, and that the fix was being applied more broadly. Microsoft was also preparing an automated process to detect and restart unresponsive servers.
The company said a review of recent changes had identified configuration updates as a potential cause of the problem, though the matter remained under investigation. At 9.41pm UTC, Microsoft said it was working through restarting and validating the affected servers and looking to speed up the process. "In parallel, we're assessing whether reverting the small number of recent configuration updates we identified could help accelerate recovery."
An earlier service degradation alert on Microsoft's site said the company was "aggressively pursuing all pathways to reduce impact" and was "restarting affected servers to return them to service". It added: "As a precaution, we've paused all deployments and configuration changes while we review recent changes for any correlation to this event."
The company said the elevated load was not caused by an overall increase in traffic. "We've determined that the elevated load isn't driven by an overall increase in traffic, but by a large number of servers being unexpectedly unavailable, which is shifting load onto the remaining infrastructure. We're investigating what's causing these servers to become unavailable while we work to safely return them to service."
Mark Gregory, associate professor in the School of Engineering at RMIT University, said the outage appeared to be affecting the network layer in Microsoft's systems and was potentially similar to the configuration issue that brought down Optus's network in 2023. He said that when some servers have configuration issues, customers are forced to share a smaller pool of servers. "If you lose part of that pool, then all of the users are trying to access the remaining servers, and that overloads them. And then they go down. And so you end up with a cascading problem."
Gregory said the incident showed companies should invest more in testing environments for configuration changes before deployment in live networks, and that businesses reliant on cloud services such as Microsoft 365 should have backup options. "Many people would keep a Gmail account, if their Microsoft goes down, they use Gmail. If Gmail goes down, they use Microsoft." Earlier, Microsoft said it had "successfully restored" problems related to Teams in North America, although some users continued to report connectivity issues.
