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

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Impact to additional Microsoft 365 services is described under MO1550821 in the Service Health Dashboard. Current status: While the servers we've returned to service initially remain healthy, we've observed that they can become unresponsive again once they begin receiving traffic. We're analyzing diagnostic data from these servers with our networking and platform experts to determine what's triggering this behavior. Building on the protective traffic management measures we've implemented, we're also temporarily limiting which types of traffic can reach the affected infrastructure in specific locations. This will help us determine whether a particular type of request is contributing to the impact and allow us to restore service incrementally. Additionally, we're rebuilding a subset of affected servers using a previous software configuration to validate whether this prevents them from becoming unresponsive, and we'll expand this approach to additional servers if we observe positive results. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Friday, November 13, 2026, at 1:00 AM UTC

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Impact to additional Microsoft 365 services is described under MO1550821 in the Service Health Dashboard. Current status: We've begun returning a portion of the affected servers to service, and they're remaining stable so far. We’ve also shifted traffic and connections to alternative pathways wherever possible and have observed limited recovery in some regions. We've streamlined our recovery process to expedite returning the remaining affected servers to service. To further support recovery, we're implementing protective traffic management measures in a specific location to reduce load on the affected infrastructure, and we'll expand these measures to additional locations if we observe positive results. As capacity is restored, users may experience a gradual recovery as their clients reconnect to the service. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Friday, November 13, 2026, at 12:00 AM UTC

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Impact to additional Microsoft 365 services is described under MO1550821 in the Service Health Dashboard. Current status: We’re continuing to pursue all available pathways to restore service, as our actions taken thus far have not provided the anticipated relief. Our investigation has found that a core process on the affected servers is becoming overwhelmed, which causes the servers to stop responding and contributes to the capacity loss. To validate a potential recovery approach, we're fully isolating a portion of our service infrastructure, restarting the affected components, and gradually reintroducing traffic to confirm our services remain stable before applying this approach more broadly. We're also analyzing traffic patterns across our entire service infrastructure and evaluating whether isolating traffic within each region may allow affected regions to recover independently. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 11:00 PM UTC

We're continuing to work through restarting and validating the affected servers to safely return them to service, and we're exploring opportunities to expedite this process. In parallel, we're assessing whether reverting the small number of recent configuration updates we identified could help accelerate recovery. Next update by: Thursday, November 12, 2026, at 10:00 PM UTC This quick update is designed to give the latest information on this issue.

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Impact to additional Microsoft 365 services is described under MO1550821 in the Service Health Dashboard. Current status: 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. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 10:00 PM UTC

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. Next update by: Thursday, November 12, 2026, at 9:00 PM UTC This quick update is designed to give the latest information on this issue.

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Impact to additional Microsoft 365 services is described under MO1550821 in the Service Health Dashboard. Current status: While we haven't identified any recent changes that correlate with the start of impact, our investigation indicates the failed health checks may originate from a specific component on the affected servers that's responsible for processing them. We're deploying a targeted change to bypass this component on a subset of servers to validate whether this restores availability, and we've taken steps to prevent additional servers from being removed from service while we work to return the affected servers to service. We're also investigating why affected servers are experiencing elevated processor usage while handling requests. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 9:00 PM UTC

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. This quick update is designed to give the latest information on this issue. Next update by: Thursday, November 12, 2026, at 8:00 PM UTC

We've determined that the elevated load isn't driven by an overall increase in traffic, but by a large number of servers being unexpectedly unavailable, which is shifting load onto the remaining infrastructure. We're investigating what's causing these servers to become unavailable while we work to safely return them to service. This quick update is designed to give the latest information on this issue.

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. This quick update is designed to give the latest information on this issue.

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Current status: We've completed redistributing Outlook connection traffic across our worldwide infrastructure, and we're moving additional traffic off the affected infrastructure to further reduce load. We're also restarting affected servers to return them to service. In parallel, we're investigating whether the elevated traffic reflects a genuine increase in requests or a shift in how traffic is being routed across our infrastructure. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 8:00 PM UTC

We've identified a pattern in which affected servers are becoming intermittently unavailable, which places additional strain on the remaining infrastructure and contributes to the connectivity issues users are experiencing. We've begun redistributing Outlook connection traffic across our broader worldwide infrastructure to help balance the load, which may result in some latency while this change is in effect, and we're reviewing recent changes to identify the source of the increased traffic. This quick update is designed to give the latest information on this issue.

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. This quick update is designed to give the latest information on this issue.

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Current status: 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. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 7:30 PM UTC

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Current status: 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. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 8:00 PM UTC

Title: Users may be unable to connect to their mailboxes in Exchange Online through multiple connection methods 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 Current status: We’re reviewing service health telemetry information to identify and isolate any errors contributing to impact for further investigation. Scope of impact: This issue may affect users attempting to access their mailbox in Exchange Online using the affected connection methods. Next update by: Thursday, November 12, 2026, at 6:00 PM UTC

Title: Potential issues accessing mailboxes via one or more connection methods User impact: Users may experience errors or failures when accessing their mailbox via one or more Exchange Online connection methods. Current status: We're investigating a potential issue and checking for impact to your organization. We'll provide an update within 60 minutes.