Microsoft acknowledged the service issues on Tuesday at 4:04 AM UTC and noted that it was impacting users in North America and other "additional regions". The issue itself was due to a glitch impacting Microsoft's infrastructure in North America. Microsoft did not clarify which additional regions were affected, but noted in an update that the Calendar application protocol interfaces used by Microsoft Teams and other services were also affected.
However at 11.05GMT Microsoft tweeted: "We've applied mitigation throughout the affected infrastructure, and we're starting to see gradual recovery." By 3.32PM Microsoft said "Availability is at 99.9%, with full restoration almost complete. We're continuing to monitor the environment to ensure full recovery."
Microsoft had earlier detailed the problem at 08:43AM on its service health page. "Current status: Users in additional regions beyond North America may experience some residual impact due to the affected portions of infrastructure in North America. We've begun observing gradual improvement from this issue for users located in some of the additional affected regions. We're continuing to perform targeted restart operations on the primarily affected infrastructure in North America in order to restore the availability of the service," it said.
Microsoft's @MSFT365Status Twitter account reported that a "recent change" was contributing to the service disruption. It has not clarified what change it was referring to.
"We've confirmed that a recent change is contributing to the cause of impact. We're working on potential solutions to restore availability of the service. Refer to EX512238 or https://portal.office.com/servicestatus for more detailed information," it noted.
The now-ended disruption to Outlook.com follows a global multi-hour outage at the end of January that impacted several Microsoft cloud services, including Microsoft Teams, Exchange Online, Outlook, SharePoint Online, OneDrive for Business, Microsoft Graph, Power BI, M365 Admin Portal, Microsoft Intune, Microsoft Defender for Cloud Apps, and Microsoft Defender for Identity.
The company said in a preliminary post-incident review that "a change made to the Microsoft Wide Area Network (WAN) impacted connectivity between clients on the internet to Azure, connectivity across regions, as well as cross-premises connectivity via ExpressRoute."
The outage happened after a command given to one of its routers in the WAN caused the router to send messages to all other rounders in the WAN to recalculate their IP tables. The particular command had not been vetted by engineers prior to deployment.