Update September 11, 18:12 EDT: Revised story and title after Microsoft's confirmation that the outage has been mitigated.
Microsoft says that it has mitigated an Exchange Online outage affecting customers worldwide, which blocked their access to emails and calendars.
"We're investigating an issue affecting a portion of infrastructure in North America, where users may be unable to access their mailbox via any Exchange Online connection method," the company said on Thursday morning.
According to user reports on DownDetector, the issue began impacting Microsoft's customers early in the morning and is causing server connection problems, affecting users who attempt to log in to their accounts and access Teams, Outlook, and Hotmail.
While the root cause has yet to be determined, Microsoft added in an admin center update that it was investigating telemetry data and working to mitigate the impacted infrastructure.
"We've collected trace logs that indicate unexpectedly high resource utilization which may be contributing to connection errors and failures for mailboxes hosted on this portion of infrastructure," Microsoft said. "We're currently working to identify and address the underlying cause of the high CPU utilization."
In a subsequent service alert update, the company said it was seeing signs of recovery after implementing a configuration change to some of the impacted infrastructure, with reduced "impact of the high service utilization on the environment."
Sixteen hours into the outage, Microsoft updated its admin center incident report to say that the incident wasn't region-specific as initially believed, and users may also "experience delays in email delivery" besides issues when accessing their mailboxes.
On Friday, around 2:00 AM, Redmond announced that it had successfully mitigated the outage after restoring most of the previously degraded infrastructure.
"Our monitoring of service health indicates that the service has returned to optimal levels. We'll continue monitoring the environment over the course of the next several hours to ensure our services remain healthy," Microsoft said.
"Our service health remains within expected thresholds as we continue our build versioning update. We'll continue to provide updates on this deployment for the next several hours as it completes," it added one hour later.
Earlier this week, Redmond resolved another issue that caused an anti-spam service to quarantine some emails and mistakenly block Exchange Online and Microsoft Teams users from opening URLs.
The company is also rolling out a fix to address an Exchange Online issue that triggers email access problems for Outlook mobile users who use Hybrid Modern Authentication (HMA).
The issue began impacting Android and iOS users on August 17, causing mailboxes to crash and preventing them from accessing their email and calendars.