Microsoft's Outlook email service remained impaired for many users on Friday morning, nearly 19 hours after the disruption began. The company indicated that its remediation efforts were yielding only gradual improvements and did not provide a timeline for full restoration.
Outage Timeline and Scope
The incident started at 16:17 GMT on Thursday, according to Microsoft's incident report for Exchange Online, the backbone of business Outlook mailboxes. The disruption spread from the Americas to Asia overnight and reached Europe as offices opened. By 11:11 GMT on Friday, Microsoft 365 Status reported that recovery work had been completed on "portions" of the affected infrastructure, with mailbox access and delayed email improving, though intermittent problems persisted for some users.
The outage primarily affected Exchange Online, impacting access via Outlook on the web, desktop Outlook, and mobile mail apps. Any Microsoft 365 feature relying on Exchange, including calendar scheduling in Teams, parts of SharePoint and Copilot, and several security tools, was also disrupted. Features independent of Exchange, such as joining Teams calls, continued to function.
Microsoft disclosed in April 2025 that it had over 430 million paid Microsoft 365 commercial seats, underscoring the scale of dependency. Downdetector logged reports from 74 countries and territories within an hour, peaking at 19,650 reports per minute across five Microsoft services, compared to fewer than 15 on a typical Thursday.
Root Cause and Recovery Challenges
Microsoft has not yet published a root cause, promising a post-incident report once services are restored. Initial status updates indicated that a number of servers became "unexpectedly unavailable," straining the remaining infrastructure. These servers failed automated health checks and were pulled from service, prompting Microsoft to pause deployments and begin restarts.
By early Friday, Microsoft noted that a process for setting up secure connections on affected servers was being overwhelmed. It also started reverting a recent network configuration change that "correlates with the start of impact," though this was not confirmed as the cause. The primary difficulty, according to Microsoft, was that recovered servers did not stay recovered; servers returned to service could become unresponsive again upon receiving traffic.
At 03:13 GMT on Friday, Microsoft said applied changes had not taken effect as expected. Later updates described a surge of calls from routing systems to back-end servers after traffic rebalancing, with machines stuck in a "bad state" blocking network changes from syncing. Engineers have been bringing capacity back slowly, limiting traffic in some regions and draining, restarting, and patching servers in stages.
Progress became evident around 07:24 GMT when Microsoft reported that a patch to an "upstream authentication/routing configuration component" was improving server stability. By 09:24 GMT, it noted "early stabilisation" and extended its email timeout to 48 hours to hold queued messages rather than bouncing them back.
Market and Operational Impact
The disruption spilled into financial markets. Bloomberg reported that traders resorted to phone, WhatsApp, and even fax for business, with trade confirmations backing up. In Singapore, some banks were unable to complete financing deals. Euronext asked listed companies to phone through corporate information because its listing teams could not receive emails. Workhorse Group postponed its earnings release, and Richemont briefly delayed its half-year results, according to NBC News.
In the UK, NHS England said staff were turning to paper records where needed, as reported by The Guardian. Japan's government reported disruption to communications between ministries.
Microsoft shares were at $512.08 in pre-market trading at 11:19 GMT on Friday, down 2.1% from Thursday's close of $523.00. The stock had touched $529.66 on Thursday before the outage widened, and closed up 0.4%.
Outside experts have expressed surprise at the duration. Gergely Orosz, a former engineer at Skype and Uber, told the BBC he "cannot recall" a business email outage taking this long to return to normal.
This article is for informational purposes only and does not constitute financial advice.
