Users may be unable to update calendar delegates in the classic Outlook desktop client

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may have been unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users could’ve managed delegate changes through Outlook on the web. Final status: Our fix deployment is complete and we’ve determined from service telemetry that this issue is resolved. Scope of impact: Impact was specific to users that were unable to update calendar delegates in the classic Outlook desktop client. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC End time: Saturday, October 3, 2026, at 12:00 AM UTC Root cause: A recent update to the service contained a regression, resulting in the impact. Next steps: -We're reviewing our validation and updating procedures to better detect and prevent issues like this prior to deployment in the future. This is the final update for the event.

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: Our fix has saturated 95 percent of the affected environment and we'll continue to monitor its progress to ensure it completes successfully. We anticipate that it'll reach full percent saturation and resolve this issue by our next scheduled communications update. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. This information may be updated as our investigation continues. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Tuesday, October 6, 2026, at 2:00 AM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We're still monitoring the deployment of our fix, and more time is needed to ensure it fully saturates across all affected environments before confirming impact remediation. We will share an updated estimate of completion once one becomes available. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. This information may be updated as our investigation continues. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Saturday, October 3, 2026, at 2:00 AM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: While the deployment of our fix will take longer than previously expected, we've confirmed that saturation is currently at 93 percent. We anticipate that deployment will reach full saturation by the time of our next scheduled communications update. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. This information may be updated as our investigation continues. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Estimated time to resolve: Our updated timeline for full remediation is Friday, October 2, 2026, at 12:00 AM UTC. Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Friday, October 2, 2026, at 2:00 AM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: Our deployment is at 62 percent saturation, and we anticipate deployment to be complete by our next scheduled update. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. This information may be updated as our investigation continues. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Estimated time to resolve: We anticipate impact to be resolved by our next scheduled update on Thursday, October 1, 2026, at 10:00 PM Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Thursday, October 1, 2026, at 10:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We’ve validated our fix and started the deployment to the affected environment. We'll provide a timeline for the complete saturation of this fix if one becomes available. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. This information may be updated as our investigation continues. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Wednesday, September 30, 2026, at 10:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We’ve validated our fix and started the deployment to the affected environment. We'll provide a timeline for the complete saturation of this fix if one becomes available. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. This information may be updated as our investigation continues. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Wednesday, September 30, 2026, at 10:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We’re in the testing and validation stages of our fix deployment procedures and we anticipate that these actions will be completed, as well as the subsequent deployment process prior to our next scheduled update. At which point, we aim to provide a remediation timeline. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Monday, September 28, 2026, at 10:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We’re in the testing and validation stages of our fix deployment procedures and we anticipate that these actions will be completed, as well as the subsequent deployment process prior to our next scheduled update. At which point, we aim to provide a remediation timeline. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Monday, September 28, 2026, at 10:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We've confirmed that reverting the responsible change is not a valid remediation pathway for this event and have instead developed a code fix to address the regression. We'll provide an update remediation timeline once we start deploying this fix. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. Start time: Wednesday, September 23, 2026, at 7:13 AM UTC Root cause: A recent update to the service contained a regression, resulting in this impact. Next update by: Saturday, September 26, 2026, at 1:30 AM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We're continuing our plan to revert the change while also looking for methods of expediting the process. Once complete, we'll validate using service telemetry to confirm recovery for all affected users. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. Next update by: Friday, September 25, 2026, at 8:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We're continuing our plan to revert the change while also looking for methods of expediting the process. Once complete, we'll validate using service telemetry to confirm recovery for all affected users. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. Next update by: Friday, September 25, 2026, at 8:00 PM UTC

Title: Users may be unable to update calendar delegates in the classic Outlook desktop client User impact: Users may be unable to update calendar delegates in the classic Outlook desktop client. More info: As a workaround, affected users can manage delegate changes through Outlook on the web. Current status: We've identified that a recently enabled change may be causing authentication failures affecting delegate permission modification requests. We're reverting the change and are validating service telemetry to confirm recovery for affected users. Scope of impact: Impact is specific to users that are unable to update calendar delegates in the classic Outlook desktop client. Next update by: Thursday, September 24, 2026, at 4:30 PM UTC

Title: Users can't update delegates in their calendar within the classic Outlook desktop client User impact: Users can't update delegates in their calendar within the classic Outlook desktop client. Current status: We've identified that a recently enabled change may be causing authentication failures affecting delegate permission modification requests. We're reverting the change and are validating service telemetry to confirm recovery for affected users. Scope of impact: Your organization is affected by this event, and users who are attempting to update delegates in their calendar within the classic Outlook desktop client are impacted. This section may be updated as our investigation continues. Next update by: Thursday, September 24, 2026, at 3:30 PM UTC

Title: Users can't update delegates in their calendar within the classic Outlook desktop client User impact: Users can't update delegates in their calendar within the classic Outlook desktop client. Current status: We've identified authentication failures affecting delegate permission modification requests. Analysis shows that affected requests are being rejected with unauthorized responses, resulting in access denied errors. We're investigating why authentication tokens are being rejected and working to identify an appropriate mitigation path. Scope of impact: Your organization is affected by this event, and users who are attempting to update delegates in their calendar within the classic Outlook desktop client are impacted. This section may be updated as our investigation continues. Next update by: Thursday, September 24, 2026, at 3:00 PM UTC

Title: Users can't update delegates in their calendar within the classic Outlook desktop client User impact: Users can't update delegates in their calendar within the classic Outlook desktop client. Current status: We're reaching out to your organization to request additional network traces and Outlook client logs to support our next troubleshooting steps. Scope of impact: Your organization is affected by this event, and users who are attempting to update delegates in their calendar within the classic Outlook desktop client are impacted. This section may be updated as our investigation continues. Next update by: Thursday, September 24, 2026, at 1:00 PM UTC

Title: Users can't update delegates in their calendar within the classic Outlook desktop client User impact: Users can't update delegates in their calendar within the classic Outlook desktop client. Current status: We're reviewing support provided logs and available service side data to refine our understanding of the underlying cause of impact so we can determine our next troubleshooting steps. Scope of impact: Your organization is impacted by this event, and users who are attempting to update delegates in their calendar within the classic Outlook desktop client are impacted. This section may be updated as our investigation continues. Next update by: Thursday, September 24, 2026, at 9:30 AM UTC

Title: Users can't update delegates in their calendar within the classic Outlook desktop client User impact: Users can't update delegates in their calendar within the classic Outlook desktop client. Current status: We're reviewing support provided logs and available service side data to refine our understanding of the underlying cause of impact so we can determine our next troubleshooting steps. Scope of impact: Your organization is impacted by this event, and users who are attempting to update delegates in their calendar within the classic Outlook desktop client are impacted. This section may be updated as our investigation continues. Next update by: Thursday, September 24, 2026, at 9:30 AM UTC

Title: Users can't update delegates in their calendar within the classic Outlook desktop client User impact: Users can't update delegates in their calendar within the classic Outlook desktop client. Current status: We're investigating a potential issue with users being unable to update delegates in their calendar within the classic Outlook desktop client, and we're checking for impact to your organization. We'll provide an update within 30 minutes.