Last Monday into Tuesday there was a serious outage that affected Outlook for Windows users. It shows how a failure on Microsoft’s server can stop new Outlook software from working at all.
About 5% of global Outlook for Windows customers were unable to open the new Outlook desktop client on Windows devices for up to 19 hours.
This directly affected their ability to access any email and calendar services through the application. That’s because Outlook for Windows is a web app that relies on Microsoft’s cloud service, even for non-Microsoft hosted mailboxes.
Microsoft says the ‘incident’ happened across 19 hours, for the US East Coast that was 6:00 PM EDT on Monday to 1:17 PM on Tuesday afternoon. (UTC time: between Monday, August 18, 2025, at 10:00 PM to the following afternoon at 5:17 PM)
Turn on the Outlook Irony feature
We enjoyed the irony of Microsoft telling affected users to use the Outlook classic desktop client to maintain access.
This is the same Microsoft that goes to any lengths pushing new Outlook on their paying customers and discouraging the more reliable and private ‘classic’ Outlook.
Final Status
Microsoft says the service has been reverted to the last known stable build and Outlook new for Windows is working again.
Scope of Impact
It was a pleasant change to see Microsoft admit that “Approximately five percent of users ” were affected.
Usually Redmond is deliberately vague about the extent of a failure. Phrases like “some users” are standard.
Is this latest incident report the sign of a new transparency at Microsoft? Or will someone get into trouble for disclosing a real fact to customers?
Some things don’t change. The ‘explanation’ of the failure was a nerd word salad with a definite wiff of Copilot AI.
“A recent deployment aimed at optimizing dependencies within the affected scenario introduced instability, resulting in application crashes and access failures.”
Source: Microsoft incident EX1137908
