Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

We're continuing to observe strong system stability across the previously impacted environment and we expect we'lll be able to confirm the issue is fully resolved by our next scheduled update.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. We'll continue to monitor service health and take any necessary actions to remediate residual impact.

Affected users may receive a 421 4.3.2 error code that states "Cannot establish session with remote server."

Current status: Our transition to the functional legacy mechanism across the affected infrastructure has completed successfully. This change will help distribute email traffic more evenly across the service and adds additional resiliency to the impacted environment. In our review of system logging we've identified a configuration issue that prevented some Exchange email delivery systems from properly distributing traffic load, which is a likely source of underlying impact. We are continuing to validate this potential root cause.

Current telemetry indicates that email sending and delivery services continue to remain stable with no observable impact. We’ll continue monitoring to ensure service stability through peak North America business hours.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Preliminary root cause: A configuration issue prevented some Exchange email delivery systems from properly distributing traffic load, reducing available capacity in several regions and causing increased traffic on remaining systems, which resulted in email delivery delays and disruptions.

Next update by: Wednesday, June 03, 2026, at 6:00 PM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. This status may change as investigation and mitigation activities progress.
Affected users may receive a 421 4.3.2 error code that states "Cannot establish session with remote server.".

Current status: We're continuing to apply a change to transition to a functional legacy mechanism across the remaining affected infrastructure. This task is progressing as expected, and we're monitoring the change as it progresses to ensure it delivers the expected resolution. We're analyzing telemetry closely, and we've observed an increase in service availability within the infrastructure that the change has been successfully applied to. In parallel, we're leading a separate investigation to identify a suspected issue which may have caused the email delivery and sending delays. We're conducting additional targeted restarts of service components and infrastructure to identify if any additional troubleshooting, or mitigation actions are required.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 4:00 PM UTC

We're closely monitoring the transition to a proven legacy mechanism across the remaining affected infrastructure, and this is progressing as expected. We're analyzing service telemetry, and we're continuing to observe an increase in service availability as this task is underway.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. This status may change as investigation and mitigation activities progress.

Affected users may receive a 421 4.3.2 error code that states "Cannot establish session with remote server.".

Current status: We're applying a change to transition to a proven legacy mechanism to mitigate impact. We've observed an increase in service availability within the infrastructure in which this change has already been completed, and we're closely tracking the progress of this change within the remaining affected infrastructure to ensure the expected resolution is achieved. Additionally, we're conducting an investigation into a potential issue which may have created the initial impact, causing email delivery and sending delays. To verify this theory, we're conducting additional troubleshooting steps, including targeted restarts, to determine if any other mitigation actions are required.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 2:00 PM UTC

As previously mentioned, we’re not seeing delays in sending and receiving email messages at this time. We're continuing to observe an overall improvement in service availability since conducting the transition to the legacy mechanism, and we're monitoring this closely as it is applied to the remaining infrastructure to ensure this progresses as expected.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. This status may change as investigation and mitigation activities progress.

Affected users may receive a 421 4.3.2 error code that states "Cannot establish session with remote server.".

Current status: We're continuing to make targeted adjustments, including a transition to a proven legacy mechanism to mitigate impact. Early reports from service telemetry indicate an improvement in service availability, and we're monitoring this closely to ensure this trend continues. In parallel, we're conducting further investigation into a potential issue which may have contributed to the email delivery and sending delays, and we're completing additional troubleshooting steps to determine if any other mitigation actions are required.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 12:00 PM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. This status may change as investigation and mitigation activities progress.

Affected users may receive a 421 4.3.2 error code that states "Cannot establish session with remote server.".

Current status: We're monitoring service health telemetry closely to ensure the targeted adjustments we've made are continuing to mitigate impact. In parallel, we're continuing to investigate a potential issue which may have contributed to the email delivery and sending delays to verify if any additional mitigation actions are required.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 10:00 AM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. This status may change as investigation and mitigation activities progress.

Current status: We’ve made progress in the investigation and identified a potential issue that may have contributed to the email delivery and sending delays. We’ve implemented targeted adjustments to improve related processing behavior and are actively monitoring the service to validate recovery and confirm continued stability.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 8:00 AM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: At this time, we’re not seeing delays in sending and receiving email messages. This status may change as investigation and mitigation activities progress.

Current status: We’ve continued monitoring traffic patterns and are currently not observing delays affecting sending or receiving email messages. We’re remaining actively engaged around the clock to monitor service health, analyze telemetry, and implement targeted actions as needed to help maintain service stability and improve overall performance.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 6:00 AM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We’ve continued making progress in our investigation to isolate the root cause and identify additional mitigation opportunities. We’re actively analyzing telemetry and traffic patterns across the affected infrastructure to help drive targeted modifications and improve overall service stability and performance.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 4:00 AM UTC

Investigation efforts remain ongoing as we continue narrowing the potential root causes and evaluating mitigation opportunities to support full service recovery.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: Our investigation efforts remain ongoing to isolate the root cause of delays affecting email sending and receiving. We’ll continue reviewing traffic distribution across the infrastructure to identify and implement targeted actions that will help restore the service to normal operations.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 2:00 AM UTC

Our investigation efforts remain ongoing as we work to determine the root cause and implement additional mitigations as needed to expedite recovery.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We’re continuing to analyze network traffic volumes to isolate the underlying cause of impact and support our recovery efforts. Email queues have stabilized at pre-incident levels, and we’ll continue monitoring traffic patterns as recovery efforts progress.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 1:00 AM UTC

We've confirmed that the backlog of message queues has returned to pre-incidents levels. We're still working to isolate the root cause and identify additional mitigations to safeguard the service environment.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We’re continuing to investigate a potential network distribution issue that resulted in certain servers receiving a higher volume of traffic than expected. While we work to confirm this as an underlying cause of impact, we are still observing steady recovery with mail queues approaching pre-incident levels.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Wednesday, June 03, 2026, at 12:00 AM UTC

We identified a potential traffic distribution issue that may have caused imbalances in mail distribution across the affected infrastructure and we're working to confirm whether this is the source of impact for this incident.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We're continuing to observe steady improvements in the reduction of message queue volume, in the portion of affected infrastructure where we completed our configuration change deployment and machine restarts. We're continuing to examine system logging and trends in mail delivery in order to isolate a root cause and identify further mitigations.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 AM UTC

Next update by: Tuesday, June 02, 2026, at 11:00 PM UTC

Our targeted restarts have reached approximately 50 percent of impacted machines and we're continuing to observe steady improvements in reducing message queue volume.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We've observed improved message throughput in the sample machines where the configuration change was deployed. We're continuing with a batched deployment of the configuration change and staggered restarts across the impacted infrastructure. We're continuing to work with highest priority to identify additional service optimizations to expedite full resolution of this issue.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 AM UTC

Next update by: Tuesday, June 02, 2026, at 10:00 PM UTC

We've deployed the flight and are restarting the service on the subset of targeted machines to activate the configuration changes. We'll closely monitor the health of the targeted machines to confirm the efficacy of the flight deployment.

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

We're preparing a flight to test configuration changes on a subset of impacted infrastructure intended to further drive down memory usage and drain message queues. If successful, we'll deploy the configuration changes broadly.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: In the subset of the affected environment where we disabled the high-memory component, we found that the improvements to the mail queues were temporary. We’re now evaluating targeted service infrastructure resets as a mitigation step, and collecting fresh system trace logs to help determine the next remediation actions.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Start time: Tuesday, June 02, 2026, at 1:03 PM UTC

Next update by: Tuesday, June 02, 2026, at 9:00 PM UTC

We've deployed the flight to disable the high memory usage component to a subset of infrastructure and are observing some improvements in the service environment. We're running additional tests to determine whether expanding this fix will result in full remediation.

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

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We've identified a service component that is consuming a high amount of system memory, which could be contributing to impact. We're preparing a flight to disable this component as a potential mitigation and expect to deploy it within the next hour.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Next update by: Tuesday, June 02, 2026, at 8:00 PM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We've received reports of impact for users in the Asia-Pacific (APAC) and Europe regions, and we've expanded our communications to encompass all potentially affected users and have updated the "Scope of impact" field of our communications to reflect this change. We're analyzing the mail queue backlog for affected regions to further our understanding of the current impact scenario and to isolate potential points of failure contributing to impact.

Scope of impact: Impact is specific to users located in North America, Europe, or the Asia-Pacific regions attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Next update by: Tuesday, June 02, 2026, at 7:30 PM UTC

Title: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays or failures in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We're reviewing additional reports from customers in the North America and Germany regions to further our understanding of the current impact scenario, to isolate error messages affected users are receiving, and to determine our next troubleshooting steps.

Scope of impact: Impact is specific to users located in North America and Germany attempting to send and receive email messages in Exchange Online who may experience delays or failures completing these actions. This section may be updated as the investigation progresses.

Next update by: Tuesday, June 02, 2026, at 6:30 PM UTC

Title: Users may be experiencing significant delays in sending and receiving email messages in Exchange Online

User impact: Users may be experiencing significant delays in sending and receiving email messages in Exchange Online

More info: Users are experiencing significant delays in sending and receiving email messages, with some email messages remaining undelivered for over an hour.

Current status: We’re analyzing support case details to determine the next steps needed to resolve this issue.

Scope of impact: Impact is specific to users located in North America and Germany attempting to send and receive email messages in Exchange Online.

Next update by: Tuesday, June 02, 2026, at 4:30 PM UTC