Microsoft Tests Health-Check Bypass During Outlook Mailbox Disruption

Microsoft said it was testing a targeted server change during a November 12 disruption to Outlook mailbox access. In its latest available update, reproduced in a University of Pennsylvania collection of Microsoft incident messages, the company said it would bypass a health-check component on a subset of servers to see whether availability returned. It had not confirmed that the test worked.

For businesses that rely on Microsoft-hosted email, the mailbox incident lists Outlook on the web, MAPI, Exchange ActiveSync and REST API as potentially affected ways to connect. A separate Microsoft 365 suite incident names Exchange Online, Teams, SharePoint Online, Microsoft 365 Copilot and Defender XDR as potentially affected services. Those lists describe possible scope; they do not establish that every listed service is unavailable to every customer.

What Microsoft found

Earlier in the investigation, Microsoft said an unexpectedly large number of unavailable servers had shifted work onto the remaining infrastructure. It said the elevated load did not reflect an overall increase in traffic. The company subsequently identified failing automated health checks as the reason servers were becoming unavailable and said it had taken steps to prevent additional servers from being removed from service.

Microsoft had not identified a recent change that correlated with the start of the disruption. In the available notices, the mailbox incident, EX1550543, and the broader suite incident, MO1550821, were both marked service degradation without an end time.

An earlier, separate Teams messaging incident, TM1550488, was marked restored at 15:53 UTC on November 12. That closure does not close the broader incident, which still listed Teams among its potentially affected services.

Administrators can check Microsoft's service-health guidance for the Health → Service health page in the Microsoft 365 admin center, where incident details can be checked for their organization.