--- title: Microsoft Outlook Down: What Really Happened? | daily.dev description: A Microsoft 365 outage on November 12th took down Outlook, along with reports of problems accessing Teams and other Microsoft 365 services. Microsoft tracked... canonical: https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Microsoft Outlook Down: What Really Happened? | daily.dev og:description: A Microsoft 365 outage on November 12th took down Outlook, along with reports of problems accessing Teams and other Microsoft 365 services. Microsoft tracked... og:url: https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh og:image: https://api.daily.dev/og/posts/KNUQIJHSh.png og:image:alt: Microsoft Outlook Down: What Really Happened? og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Microsoft Outlook Down: What Really Happened? **[ByteMonk](https://daily.dev/sources/bytemonk)** · 8 min read · 0 upvotes · 0 comments ## Summary A Microsoft 365 outage on November 12th took down Outlook, along with reports of problems accessing Teams and other Microsoft 365 services. Microsoft tracked the Exchange Online incident as EX155043 and reported that servers were failing automated health checks, leaving the remaining infrastructure with more work. A later finding said a process responsible for establishing secure connections was getting overwhelmed. Microsoft redistributed traffic, restarted or repaired affected servers, tested an alternative health-check method, and extended its message timeout window to 48 hours to preserve delayed emails during recovery. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What caused the Microsoft Outlook outage on November 12? Servers unexpectedly dropped out of service after failing automated health checks, shifting their load onto remaining servers and triggering a cascading failure. Microsoft later found a process responsible for establishing secure connections was getting overwhelmed, causing affected servers to stop processing requests. The incident was tracked internally as Exchange Online incident EX155043 and also disrupted Teams, SharePoint, Copilot, and Microsoft Graph. _Following outage postmortems like this on daily.dev helps you design services that survive cascading failures._ ### How does a cascading failure spread across a server cluster? When some servers in a cluster go down, remaining servers absorb their traffic and exceed their comfortable capacity, causing timeouts and failed health checks. Those newly failing servers then get pulled from rotation too, pushing even more load onto fewer machines. In a 10-server example handling 10,000 requests per second, losing 3 servers pushes the remaining 7 to about 1,400 requests per second each, well above a 1,200 struggle threshold. _Understanding this failure pattern on daily.dev helps when you're debugging why redundancy didn't save your system._ ### How long did Microsoft extend email delivery timeouts during the Outlook outage recovery? Microsoft extended its message timeout window to 48 hours to help preserve emails while backlogs built up during the outage were cleared. This gave the recovery effort two separate tasks: restoring server capacity and safely processing the accumulated backlog of delayed messages without losing them. _Tracking real recovery timelines on daily.dev helps you set realistic SLAs when your own systems face backlogs._ ## Similar posts on daily.dev - [Microsoft 365 hit by outage, preventing access to emails and files](https://daily.dev/posts/microsoft-365-hit-by-outage-preventing-access-to-emails-and-files-irywo2f8j) · TechCrunch · 0 upvotes · 0 comments - [Microsoft Exchange Online outage causes email delays, failures](https://daily.dev/posts/microsoft-exchange-online-outage-causes-email-delays-failures-iugzphfkb) · BleepingComputer · 0 upvotes · 0 comments --- Tags: [#microsoft](https://daily.dev/tags/microsoft), [#distributed-systems](https://daily.dev/tags/distributed-systems) [View this post on daily.dev](https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Microsoft Outlook Down: What Really Happened?","url":"https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh"},"datePublished":"2026-11-13T18:46:01.247Z","dateModified":"2026-11-13T22:21:22.686Z","description":"A Microsoft 365 outage on November 12th took down Outlook, along with reports of problems accessing Teams and other Microsoft 365 services. Microsoft tracked...","image":"https://i.ytimg.com/vi/tFau550Piy4/sddefault.jpg","thumbnailUrl":"https://i.ytimg.com/vi/tFau550Piy4/sddefault.jpg","isAccessibleForFree":true,"articleSection":"ByteMonk","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"ByteMonk","logo":"https://media.daily.dev/image/upload/s--n36_-Uve--/f_auto/v1752670492/logos/bytemonk","url":"https://daily.dev/sources/bytemonk"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"microsoft,distributed-systems","timeRequired":"PT8M","video":{"@type":"VideoObject","name":"Microsoft Outlook Down: What Really Happened?","description":"A Microsoft 365 outage on November 12th took down Outlook, along with reports of problems accessing Teams and other Microsoft 365 services. Microsoft tracked...","thumbnailUrl":"https://i.ytimg.com/vi/tFau550Piy4/sddefault.jpg","uploadDate":"2026-11-13T18:46:01.247Z","duration":"PT8M","url":"https://api.daily.dev/r/KNUQIJHSh","embedUrl":"https://www.youtube.com/embed/tFau550Piy4"}} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"ByteMonk","item":"https://daily.dev/sources/bytemonk"},{"@type":"ListItem","position":3,"name":"Microsoft Outlook Down: What Really Happened?"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/microsoft-outlook-down-what-really-happened--knuqijhsh#faq","mainEntity":[{"@type":"Question","name":"What caused the Microsoft Outlook outage on November 12?","acceptedAnswer":{"@type":"Answer","text":"Servers unexpectedly dropped out of service after failing automated health checks, shifting their load onto remaining servers and triggering a cascading failure. Microsoft later found a process responsible for establishing secure connections was getting overwhelmed, causing affected servers to stop processing requests. The incident was tracked internally as Exchange Online incident EX155043 and also disrupted Teams, SharePoint, Copilot, and Microsoft Graph. Following outage postmortems like this on daily.dev helps you design services that survive cascading failures."}},{"@type":"Question","name":"How does a cascading failure spread across a server cluster?","acceptedAnswer":{"@type":"Answer","text":"When some servers in a cluster go down, remaining servers absorb their traffic and exceed their comfortable capacity, causing timeouts and failed health checks. Those newly failing servers then get pulled from rotation too, pushing even more load onto fewer machines. In a 10-server example handling 10,000 requests per second, losing 3 servers pushes the remaining 7 to about 1,400 requests per second each, well above a 1,200 struggle threshold. Understanding this failure pattern on daily.dev helps when you're debugging why redundancy didn't save your system."}},{"@type":"Question","name":"How long did Microsoft extend email delivery timeouts during the Outlook outage recovery?","acceptedAnswer":{"@type":"Answer","text":"Microsoft extended its message timeout window to 48 hours to help preserve emails while backlogs built up during the outage were cleared. This gave the recovery effort two separate tasks: restoring server capacity and safely processing the accumulated backlog of delayed messages without losing them. Tracking real recovery timelines on daily.dev helps you set realistic SLAs when your own systems face backlogs."}}]} ```