M365 Exchange Online mailbox connection issues

Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods.

Other potentially impacted services include: – Exchange Online – Microsoft Teams – SharePoint Online – Microsoft Copilot 365 – Microsoft Defender XDR – Microsoft Planner – Microsoft To Do

Affected services: Exchange Online
Status: Service degradation
Start time: Nov 12, 2026, 10:17 AM CST

User impact
Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods.

More info
Potentially impacted connection methods may include:
– Outlook on the web
– Messaging API (MAPI)
– Exchange ActiveSync (EAS)
– Representational State Transfer (REST) API

Scope of impact:
This issue may affect users in North America, Latin America, Canada, and Chile.

11/12/2026 12:21 PM - From Microsoft:
Nov 12, 2026, 11:45 AM CST
We’ve identified connectivity errors occurring on a portion of Exchange Online infrastructure which receives and processes users’ access requests. We’ve collected networking logs to review to further investigate the source of these errors and confirm our next steps.
Next update by:
Thursday, November 12, 2026 at 2:00 PM CST

11/12/2026 12:22 PM - Nov 12, 2026, 12:19 PM CST

11/12/2026 12:22 PM - Our investigation indicates that the affected infrastructure is experiencing elevated load across multiple regions, which is contributing to the connectivity errors users are receiving. We’re restarting and repairing impacted portions of this infrastructure to mitigate impact and are evaluating options to redirect traffic to alternate infrastructure to expedite relief.

11/12/2026 12:34 PM - Nov 12, 2026, 12:29 PM CST
We’ve redirected a portion of traffic away from the affected infrastructure in the US to reduce load, and we’re evaluating doing the same in other regions. While Exchange is the primarily impacted service, we’ve identified additional Microsoft 365 services that are also experiencing connectivity issues, and we’re communicating those issues under MO1550821 on the Service Health Dashboard.

11/12/2026 12:34 PM - Potentially impacted services include: – Exchange Online – Microsoft Teams – SharePoint Online – Microsoft Copilot 365 – Microsoft Defender XDR

11/12/2026 12:51 PM - Nov 12, 2026, 1:06 PM CST
We’re aggressively pursuing all pathways to reduce impact. As a precaution, we’ve paused all deployments and configuration changes while we review recent changes for any correlation to this event. We’ve also moved traffic for some services off the affected infrastructure to further reduce load, with additional traffic moves in progress.

11/12/2026 1:22 PM - Nov 12, 2026, 1:43 PM CST
We’ve identified that affected servers are failing automated health checks, which is causing them to become unavailable, and this may be related to a recent change to a component involved in processing those checks. We’re testing a configuration change on a subset of affected infrastructure to address this and, if successful, will apply it more broadly. In parallel, we’ve initiated a restart of all affected servers and are validating whether they’re successfully returning to service.

11/12/2026 2:40 PM - Nov 12, 2026, 2:38 PM CST
While validating the targeted change, we found that many of the affected servers have become unresponsive, which is preventing us from confirming whether the change is effective. We’re restarting these servers so they can come back online with the change in place, and we’re applying the same approach to additional affected portions of our infrastructure to validate the results more broadly.

11/12/2026 2:47 PM - Nov 12, 2026, 2:56 PM CST

11/12/2026 2:47 PM - We’ve validated an alternative health check method on a subset of affected infrastructure, which allowed the servers in that subset to successfully return to service, and we’re now applying this change more broadly. We’re also preparing an automated recovery process to continually detect and restart unresponsive servers, to help keep them in service while this change progresses. Additionally, our review of recent changes has identified a small number of configuration updates that we’re investigating as potential contributors to the issue.