Microsoft online services in North America including Microsoft 365, Outlook and Teams experienced disruption for more than five hours on Thursday, with Microsoft attributing the problems to “a number of servers being unexpectedly unavailable“.

Problems with Outlook peaked at around 1 p.m. ET (9 p.m. GMT) and were gradually declining as of 6 p.m. ET (2 a.m. GMT), according to user reports on Downdetector.

Exchange services affected

“We are aware and investigating reports of users experiencing issues with Microsoft 365 services, primarily issues with Exchange Online,” said Microsoft in a statement.

The company said its efforts to restore services were gradually becoming successful.

The disruption was caused by the unexpected unavailability of some servers, which created a strain on the remaining infrastructure, Microsoft said.

At about 4 p.m. ET, Microsoft said it had used an alternative health check method that allowed a subset of services to successfully return to service.

Gradual recovery

It said automated recovery systems were being prepared to maintain service stability while it introduced the change more broadly.

Microsoft said on Thursday evening that it hadn’t identified any recent change that correlated with the beginning of the disruption.

“However, our investigation indicates the failed health checks may originate from a specific component that’s responsible for processing server health,” it said.

“We’ve taken steps to prevent additional servers from being impacted while we work to return the affected servers to service.”