Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may have been unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may have replied or replied all to email messages through user inboxes. Final status: Our fix deployment has completed, and we've confirmed that this has successfully resolved the event through service health telemetry. Scope of impact: This issue may have impacted any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC End time: Wednesday, October 28, 2026, at 7:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that required that attribute, which was resulting in this impact. Next steps: - We're reviewing the responsible change to better understand what led to the error that resulted in impact and prevent similar future occurrences. This is the final update for the event.

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We're continuing to monitor the completion of our fix deployment and expect clarity into its remaining timeline to be available by our next scheduled communication update. Scope of impact: This issue may impact any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Wednesday, October 28, 2026, at 8:30 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: The fix has deployed to around 95 percent of the infrastructure. While the deployment progress has slowed, it's within our expectations as the final infrastructure unit deployments usually take more time. A majority of users are likely no longer seeing the issue as they've received the fix. We'll continue to monitor the deployment process to ensure it completes successfully. Scope of impact: This issue may impact any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Tuesday, October 27, 2026, at 8:30 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: Our fix remains 93 percent saturated, and we're continuing to monitor as the deployment progresses, albeit at a slow pace as it nears completion. We'll aim to provide a timeline for remediation as one becomes available. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Thursday, October 22, 2026, at 8:30 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: Our fix remains at 93 percent, as the remaining saturation is expected to take extra time, per process. We're continuing to monitor progress toward completion and confirm that impact is remediated for all users. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Monday, October 19, 2026, at 8:30 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: Our fix deployment has completed in the initial deployment rings and has started saturating in affected customers' environments. Our monitoring has validated that our fix has completed approximately 93 percent of the deployment, and we'll provide an updated timeline for it completing once it has become available. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Thursday, October 15, 2026, at 8:30 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: Our monitoring of the deployment is ongoing as it propagates through earlier deployment rings prior to beginning saturation to affected customers’ environments. We'll provide a projected timeline for remediation based on the rate of progress as one becomes available. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Wednesday, October 14, 2026, at 8:30 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We're continuing to monitor and track the saturation percentage of the fix deployment to ensure it reaches 100 percent in all affected environments. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Tuesday, October 13, 2026, at 8:00 PM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We've validated that the previously identified recent change that removed an attribute component from the service process that requires that attribute, has led to impact. We're continuing to monitor our fix deployment; however, this process is taking longer than anticipated. The current fix saturation is at roughly 90 percent, and we'll provide a timeline of completion once it becomes available. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Start time: Tuesday, September 8, 2026, at 11:00 PM UTC Root cause: A recent change made to the service removed an attribute component from the service process that requires that attribute, leading to impact. Next update by: Tuesday, October 13, 2026, at 1:00 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We're continuing to monitor the deployment of our fix as it progresses, and we'll aim to provide a timeline of completion by our next scheduled communications. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Next update by: Thursday, October 8, 2026, at 1:00 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: Our fix deployment is nearing completion, and most affected users should now be experiencing recovery. We’ll continue monitoring the deployment to ensure it completes and restores service for all affected users. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 1:00 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: Our fix deployment is nearing completion, and most affected users should now be experiencing recovery. We’ll continue monitoring the deployment to ensure it completes and restores service for all affected users. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 1:00 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We've initialized our fix deployment to the affected environment and anticipate the impact should be fully remediated by our next scheduled update. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Next update by: Tuesday, October 6, 2026, at 1:00 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We've completed the validation of our planned fixes and are completing the final stages to prepare them for deployment. We anticipate that users will see progressive relief beginning Monday, September 28, 2026, up to our planned remediation date of Friday, October 2, 2026. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Next update by: Tuesday, September 29, 2026, at 1:00 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We're conducting verification and validation testing on our proposed remediation to ensure that it will safely and effectively remediate this issue. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. This information may be updated as our investigation continues. Next update by: Saturday, September 26, 2026, at 12:30 AM UTC

Title: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users may be unable to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. More info: As a temporary alternative, impacted users may reply or reply all to email messages through user inboxes. Current status: We've identified a recent code change which we believe may be responsible for this issue and are developing a targeted fix to verify. Scope of impact: This issue impacts any user attempting to reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. Next update by: Friday, September 25, 2026, at 11:00 PM UTC

Title: Users can't reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users can't reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. Current status: We're reviewing support case details to better understand the impact scenario and determine our next steps. Scope of impact: Your organization is impacted by this event and all users attempting to reply to email messages sent to Microsoft 365 groups are affected. Next update by: Friday, September 25, 2026, at 6:00 PM UTC

Title: Users can't reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online User impact: Users can't reply or reply-all to email messages sent to Microsoft 365 groups in Exchange Online. Current status: We're investigating a potential issue with Exchange Online and checking for impact to your organization. We'll provide an update within 30 minutes.