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Microsoft Outlook outage drags on as retry cascade overwhelms Exchange servers
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Microsoft's Outlook service has been down for most of a workday, and the company cannot simply restart it because millions of devices retrying email would hit servers simultaneously and cause another collapse. The outage began when a core process on many Exchange servers started choking at once, reportedly after a rollout spiked CPU usage and pushed racks into maintenance mode. Automated health checks pulled failing servers out of rotation, but users' Outlook apps kept retrying, shifting load to neighboring servers and causing them to fail too. Microsoft first tried spreading connection traffic across its global infrastructure, but said nothing had provided the anticipated relief. Its current plan walls off a slice of servers, restarts them, and lets traffic back in a trickle to avoid a thundering herd. Microsoft said it is seeing positive results. A similar retry-driven outage hit Outlook and Teams in November 2024.
Source report
Outlook has been broken for most of a workday, and Microsoft can't just switch it back on. Every phone and laptop that's been retrying email since this morning would hit the servers in the same second and knock them right back over.
That's pretty much how it broke in the first place.
How the Cascade Began
Every Exchange server gets watched by an automated health check, and if it fails, it gets yanked out of rotation so nobody's email lands on a broken machine. With one bad server, that's great design. Today a core process on a whole bunch of them started choking at once (Tom Warren reports a rollout spiked CPU and pushed racks into maintenance mode), and Microsoft's own update says servers were failing those health checks and dropping out.
The Retry Feedback Loop
So follow one person's inbox. Their server gets pulled, but their Outlook app doesn't know or care. It just keeps retrying, and those retries land on the next server over, which was already running hot from everybody else's retries, so it fails its check too and dumps its users onto the next one. Now multiply that by a product sold to 450 million paid seats.
The system that removes sick servers kept making the healthy ones sick.
Microsoft's Response
Microsoft's first big move was spreading connection traffic across its worldwide infrastructure. A few hours later it posted that nothing so far had "provided the anticipated relief," which makes sense once you see the chain, because spreading the load in a cascade also spreads whatever's killing the servers.
That's why tonight's plan looks so slow. They're walling off a slice of servers, restarting them, letting traffic back in a trickle at a time, and only expanding once it holds. Skip that step and you get what engineers call a thundering herd, where the fresh servers get trampled before they finish warming up. Earlier this evening Microsoft finally said it's seeing positive results.
A Familiar Failure
This has happened to them before, by the way. In November 2024, Outlook and Teams went down after a change caused "an influx of retry requests routed through servers."
A whole workday later, the hardest part of fixing Outlook is that millions of inboxes are all trying to help.
Source
aakashguptaNeutral / independent
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