From 15:52 UTC on 25 September, users of Exchange Online found they could not reply or reply-all to messages sent to Microsoft 365 groups. The fault sat on Microsoft's side, and as of the last update it was still being worked on, with a full fix not expected until October.
Started
15:52 UTC
Duration
Not closed by vendor
Source
IsDown
Next time
Microsoft 365 is degraded. Is your product working?
An incident on their side does not always mean an outage on yours, and the only way to know is to already be watching your own endpoints. Set that up in under a minute and you will be notified the next time your own site is affected. And there will be a next time.
50 monitors for your own websites. No card.
Affected components: Exchange Online
What happened?
Microsoft's own status feed shows a single incident starting at 15:52 UTC, affecting Exchange Online's handling of replies to Microsoft 365 group emails. The first update, four minutes later, said Microsoft was investigating potential impact. By 16:22 Microsoft confirmed the scope: any user attempting to reply or reply-all to a group email was affected. At 20:57 Microsoft said it had identified a recent code change it believed was responsible, and offered a workaround, replying through individual inboxes instead of the group thread. By 22:33 Microsoft was testing a proposed fix. At 00:24 the next day, Microsoft said validation of the fix was complete but warned that users would only see "progressive relief" from 28 September, with full remediation not planned until 2 October. The last update shown, at 23:56, said deployment of the fix had begun to the affected environment. That is six updates across more than eight hours, and the incident was still open when this record was last updated, with the next update not promised until 6 October. Separately, the user reports logged on this day cover a spread of other symptoms, Planner failing to load, Power Apps unable to pull SQL data, slow SQL connectors, and an admin centre that would not save changes, across the US, UK, France, Portugal and the UAE. None of those reports map cleanly onto the Exchange Online reply issue Microsoft described, and Microsoft's own filing does not mention Planner, Power Apps or the admin centre at all.
Learning
This incident sat inside Exchange Online's mail processing logic, the part that decides what happens when you hit reply-all on a group message, not in the basic ability to send and receive mail. A simple uptime check against Exchange Online's main endpoints would have kept reporting green throughout, because the service was reachable and most mail flow worked, only this one reply path was broken. That is exactly the kind of fault that lives below the line an external ping can see: the connection is fine, the login works, but one specific workflow inside the product fails for everyone trying to use it. Microsoft's own account, including a fix timeline stretching into October, is the only source for what actually went wrong here, since the failure never showed up as a connectivity problem. If you run a product that depends on Microsoft 365 groups for collaboration, your own monitoring of whether Exchange is reachable would not have caught this, because reachability was never the issue. Your monitoring covers your half, UptimeRobot now watches the provider's half too, so next time you know which side broke without guessing.
