Microsoft 365 Service Incident and Separate Windows/Outlook Update Issues
Microsoft reported a service incident impacting Microsoft 365 core services, with users experiencing connectivity issues and service degradation across Exchange Online, Microsoft Teams, and the broader M365 suite. The incident was tracked as MO1220495, with Microsoft stating it was still in the investigating/diagnostic phase and providing no estimated time to resolution; organizations were directed to monitor the Microsoft 365 Service Health Dashboard for tenant-specific impact details.
Separately, Microsoft published guidance for a client-side stability issue where the classic Outlook desktop app can freeze/hang after installing recent Windows security updates (notably KB5074109, and also KB5073724), particularly affecting POP accounts and scenarios where Outlook PST files are stored on cloud-backed storage such as OneDrive. Recommended mitigations included using webmail, moving PST files off OneDrive, or uninstalling the problematic updates while Microsoft investigates. A third item describing “Microsoft data breach” and “zero-day vulnerabilities” is largely a repackaged Patch Tuesday/vulnerability roundup and does not substantively align with the M365 service incident or the Outlook-freeze regression.
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How this story unfolded
4 events from the most recent confirmed update back to the earliest known activity.
Microsoft links M365 disruption to possible third-party networking issue
In an 11:11 PM IST update on the same day, Microsoft said a possible third-party networking issue might be affecting access to Microsoft 365 services, including Teams and Outlook for some users. Engineers were still investigating and had not yet provided an estimated time to resolution.
Microsoft 365 service incident begins affecting Exchange Online and Teams
Microsoft reported that incident MO1220495 began at 10:15 PM IST, causing connectivity issues and service degradation across Exchange Online, Microsoft Teams, and other Microsoft 365 services. At the time, the company had not confirmed whether the disruption was global or region-specific.
Microsoft releases workaround guidance for Outlook hangs
Microsoft published temporary mitigations for the Outlook issue, recommending webmail, moving PST files off OneDrive, or uninstalling KB5074109/KB5073724 while warning that removing security updates increases security risk. The company said Windows and Outlook teams were investigating and had not provided a fix timeline.
January Windows security updates trigger Outlook freezing issues
After users installed the January 2026 Windows security updates, Microsoft identified a problem causing the classic Outlook desktop client to freeze or hang, especially for POP account users on Windows 11 25H2/24H2. The issue was also linked to apps accessing cloud-backed storage such as OneDrive or Dropbox, including Outlook PST files stored on OneDrive.
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Microsoft cloud service disruptions affecting Microsoft 365, Exchange Online, and Windows Update/Store
Microsoft reported multiple **service-impacting incidents** across its cloud ecosystem. Administrators in **North America and Canada** experienced an outage and degraded performance in the **Microsoft 365 admin center**, with some users also unable to access the *M365 app* or raise support tickets; Microsoft said it was analyzing telemetry, usage patterns, and **CPU utilization**, and reviewing user-provided **HAR files** to isolate the root cause. Separately, **Exchange Online** quarantined legitimate messages after an updated **URL rule** incorrectly marked some URLs as phishing, disrupting email flow for affected customers while Microsoft worked to release quarantined mail and unblock legitimate URLs. In another disruption, Microsoft attributed **Windows Update** and **Microsoft Store** failures/timeouts (notably impacting Windows 11 users) to a **utility power interruption** at a **West US datacenter**, which cascaded into issues with **Azure storage clusters** supporting content delivery; backup power engaged and power was later stabilized, but service recovery required additional remediation beyond restoring electricity.
Mar 21, 2026
Microsoft Fixes Outlook and Exchange Online Email Access Disruptions
Microsoft moved to resolve two separate email access problems affecting Outlook and Exchange Online users. In Exchange Online, a newly introduced **virtual account** change intermittently blocked some users from accessing cloud mailboxes through Outlook mobile apps and the new Outlook for Mac, prompting Microsoft to classify the issue as a service outage under `EX1256020`. After infrastructure restarts failed to restore access, the company identified the change as the root cause and began disabling it across affected environments. Microsoft also fixed a classic Outlook bug that caused Gmail and Yahoo accounts to stop synchronizing, triggering errors `0x800CCC0F` and `0x80070057` after users stopped receiving sign-in prompts. The company said the service-side fix was deployed, but some users could continue seeing sync failures until their OAuth token expires, such as after a password change. Microsoft published a temporary workaround to force a new sign-in prompt and noted it is still investigating other Outlook-related problems, including Exchange Web Services group creation failures and a Teams Meeting Add-in issue that can make classic Outlook unusable.
Apr 28, 2026
Microsoft Out-of-Band Update KB5078127 Fixes Windows 11 Cloud File Access and Outlook PST Freezes
Microsoft released a second January out-of-band Windows update, **KB5078127**, to remediate issues introduced by the January Patch Tuesday security update (**KB5074109**) that disrupted cloud-backed workflows on Windows 11. Reported impacts included applications being unable to open or save files to cloud storage providers such as **OneDrive** and **Dropbox**, with a common enterprise failure mode involving **Outlook** hanging when **PST** files were stored on OneDrive; users often had to terminate processes or reboot to recover. KB5078127 is a cumulative OOB release that rolls up prior January fixes, including the earlier out-of-band update (**KB5077744**) and the security/quality improvements from KB5074109, and it also includes a bundled **Servicing Stack Update** (**KB5071142**) intended to improve update reliability. The update is available via **Windows Update** and the **Microsoft Update Catalog**; organizations with automatic updates enabled should receive it automatically, while IT teams can expedite deployment through standard Windows update management workflows.
Mar 21, 2026