From Microsoft article EX1464935 issued to Exchange Admins

We've confirmed that the authentication component issue impacts other
services beyond Exchange Online. For more information regarding other
impact scenarios, please see: MO1465074.

We've opted to keep this post (EX1464935) open as Exchange Online
remains the most predominantly impacted service.

Users may experience a variety of symptoms related to this event,
including but not limited to:
- Delays, failures, or incomplete results when searching for content
within Exchange Online mailboxes.
- Delays or failures when sending or receiving email messages.
- Authentication-related errors when accessing Exchange Online services.
- Difficulties accessing or performing actions within Exchange
administration experiences.
- Intermittent failures affecting mailbox operations and message
delivery work
- Delays or failures when sending or receiving email messages via
Outlook for iOS and Android.
- Users may be unable to use room finder in Outlook calendars.

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There was a major Microsoft 365/Outlook outage spanning August 31 into September 1, 2026, and it affected both Exchange Online and Outlook users.

Microsoft's incident was EX1464935, with the broader Microsoft 365 incident tracked as MO1465074. Microsoft lists the start as August 31 at 15:08 UTC (11:08 AM EDT). The root cause was an issue in a core authentication configuration used by multiple Microsoft 365 services.

Reported symptoms included delayed
or failed sending/receiving email, Outlook/Exchange authentication
errors, mailbox-operation failures, degraded search, Exchange Admin
problems, and Outlook iOS/Android mail failures. The
authentication problem also affected services beyond Exchange Online,
including portions of SharePoint, OneDrive, Microsoft 365 Copilot, and
other Microsoft 365 workloads.

The important point for September 1
is that this wasn't just a short Outlook.com hiccup. Recovery continued
well into Tuesday. By around midday September 1, Microsoft reported
that core Exchange Online mail flow and search had recovered, while it was still validating and remediating other affected Microsoft 365 infrastructure.

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