Microsoft Exchange Online outage blocked mailbox access before Microsoft restored service

Microsoft has restored an Exchange Online outage that blocked some users from opening their mailboxes and calendars through several connection methods on March 16, 2026. The company said the disruption affected Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols before service recovered after several hours.

The incident was tracked as EX1253275 in the Microsoft 365 admin center. According to Microsoft’s final status update, the outage started at 9:30 AM UTC and ended at 1:30 PM UTC on March 16. Microsoft said users may have seen errors or failures when trying to access their mailbox, and it tied the disruption to an issue involving supporting network infrastructure.

Microsoft’s early updates suggested the impact first showed up in Europe before the company broadened the description to multiple Exchange Online connection methods. During the incident, Microsoft said telemetry indicated signs of recovery, but users continued reporting access problems until the company later confirmed that service had remained healthy for an extended period.

The Exchange issue did not arrive alone. On the same day, Microsoft also investigated a separate problem tracked as MO1253428 that affected Office.com and the Microsoft 365 Copilot web sign-in page. A public mirror of the service health notice shows Microsoft later restored that issue as well and said high traffic on supporting infrastructure slowed request processing for some users.

For businesses that depend on Exchange Online, the outage hit at a bad time because it disrupted both email access and calendar availability. Microsoft’s final incident note said it will publish a preliminary post-incident report within two business days and a final post-incident report within five business days, which means admins should get a fuller root cause explanation after the immediate recovery phase.

What happened

Microsoft first acknowledged the Exchange Online issue as a mailbox access problem affecting one or more connection methods. Later updates expanded the impact to Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online protocols, showing that this was broader than a single client or region.

In its final update, Microsoft said an underlying issue involving supporting network infrastructure caused a service availability degradation across Exchange Online connection methods. The company has not yet published the full root cause, but it has already confirmed the outage was infrastructure-related rather than a client-side bug in Outlook.

Exchange Online outage summary

What users were told to expect

Microsoft did not publish a workaround for Exchange Online users during the outage. For the separate Copilot and Office.com issue, it advised customers to use application-based Copilot services such as the Copilot desktop app, Copilot in Teams, or Copilot in Office apps while engineers worked on the web access problem.

Admins should now watch for Microsoft’s post-incident report, especially if their organization saw knock-on effects in Outlook desktop, mobile sync, or calendar access. The service health notes show Microsoft plans to provide more detail on what failed inside the supporting network layer and what it will change to reduce the chance of a repeat outage.

Key points for Microsoft 365 admins

• Exchange Online service has been restored.
• The outage affected mailbox and calendar access through multiple connection methods.
• Microsoft blamed supporting network infrastructure for the degradation.
• A separate Office.com and Copilot web sign-in issue also occurred the same day.
• Microsoft plans to issue a post-incident report with fuller root cause details.

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