Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may have been intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may have intermittently impacted some users who attempted to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Final status: We've monitored the service for an extended period of time and confirmed that error counts have remained below pre-incident levels, confirming remediation. Scope of impact: Impact may have occurred for some users intermittently and users may have been unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. Start time: Thursday, March 12, 2026, at 12:00 AM UTC End time: Monday, March 30, 2026, at 8:00 PM UTC Root cause: A recent update intended to enable new Virtual Account functionality introduced a routing conflict that was preventing the Outlook mobile apps or the new Outlook for Mac desktop client from syncing to the Exchange service as expected, leading to impact. Next steps: - We're reviewing the impacting service update to determine why the routing conflict wasn't identified through our pre-deployment testing processes, so that we can prevent similar instances of impact from occurring in the future. - Additionally, we're implementing additional optimizations on the service and client-sides to further reduce the error counts and make the service more robust. This is the final update for the event.

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: Our telemetry indicates that disabling the impacting updates has significantly reduced the number of errors. We're continuing to monitor the environment to validate no additional mitigation actions are needed. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent update intended to enable new Virtual Account functionality introduced a routing conflict that’s preventing the Outlook mobile apps from syncing to the Exchange service as expected, leading to impact. Next update by: Wednesday, April 1, 2026, at 6:00 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We're continuing to monitor the service health telemetry to ensure failure rates continue to improve while we work on the enablement of our secondary mitigation progresses. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent update intended to enable new Virtual Account functionality introduced a routing conflict that’s preventing the Outlook mobile apps from syncing to the Exchange service as expected, leading to impact. Next update by: Monday, March 30, 2026, at 6:00 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We’re performing an extended period of monitoring the service health telemetry and our review indicates that the failure rates are continuing to reduce as the enablement of our secondary mitigation progresses. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent update intended to enable new Virtual Account functionality introduced a routing conflict that’s preventing the Outlook mobile apps from syncing to the Exchange service as expected, leading to impact. Next update by: Friday, March 27, 2026, at 8:00 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We're monitoring our service health telemetry to determine whether the failures reduce as our efforts to enable our secondary mitigation progress. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent update intended to enable new Virtual Account functionality introduced a routing conflict that’s preventing the Outlook mobile apps from syncing to the Exchange service as expected, leading to impact. Next update by: Friday, March 27, 2026, at 1:30 AM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We're continuing the process of enabling the previously mentioned secondary configuration change to resolve this issue within the remaining impacted mailboxes. In parallel, we’ve identified further details regarding the root cause of the impact, and the Root cause section of this communication has been adjusted to reflect our findings. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent update intended to enable new Virtual Account functionality introduced a routing conflict that’s preventing the Outlook mobile apps from syncing to the Exchange service as expected, leading to impact. Next update by: Thursday, March 26, 2026, at 7:00 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We've completed disabling the impacting change and our telemetry shows significant improvement. We're in the process of enabling a subsequent configuration to remediate the remaining mailboxes. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact. Next update by: Thursday, March 26, 2026, at 7:00 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: While error trends continue to reduce, we suspect additional actions may be necessary to fully remediate impact. We've identified two additional actions to assist with expediting impact resolution. Firstly, we're disabling the impacting change more broadly than initially, given we've seen no adverse effects for the subset of users that it's been disabled for already. Secondly, we've identified another change which was enabled as a pre-requisite for enabling the impacting change. We're working to disable this change to assist with remediation. We expect to have more insight into timelines for completing these actions and impact resolution with our next scheduled communication update. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact. Next update by: Wednesday, March 25, 2026, at 6:30 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We've disabled the change intended to introduce a new virtual account for a subset of affected users to test and monitor for remediation. After monitoring error trends for approximately 24 hours, we're seeing improvement, but the impact isn’t fully remediated at this time. We're continuing to monitor error trends to confirm the trend is decreasing while we review what additional changes we can implement to further improve availability and fully remediate impact. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact. Next update by: Tuesday, March 24, 2026, at 6:30 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: After further assessment, we’ve confirmed that the identified change within the Exchange Online service intended to introduce a new virtual account resulted in impact. To remediate this issue, we’re disabling the change across the affected environments. Once available, we’ll provide a resolution timeline. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact. Next update by: Monday, March 23, 2026, at 6:30 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: While we’ve previously identified that restarting the affected infrastructure isn’t a sufficient fix for this issue on a wide scale; we've confirmed that it does provide immediate temporary relief. We're continuing to implement the temporary fix as needed while we test a possible long-term solution to resolve the underlying issue for all impacted users. Furthermore, we're performing a deeper analysis into understanding the specifics of the root cause, and we'll provide further details once they are identified. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Next update by: Saturday, March 21, 2026, at 7:00 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We’ve identified that restarting the affected infrastructure isn’t a sufficient fix for this issue and we’re continuing our investigation to identify and validate an appropriate remediation action. Once available, we’ll provide a timeline for resolution. Scope of impact: Impact may occur for some users intermittently and users may be unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Next update by: Friday, March 20, 2026, at 6:30 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes in some connection methods User impact: Users may be intermittently unable to access their Exchange Online mailboxes in some connection methods. More info: This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client. Current status: We've identified that a recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact. To remediate this, we’re restarting the affected infrastructure and monitoring the progression to ensure successful resolution. Scope of impact: Impact may occur for some users intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Start time: Thursday, March 12, 2026, at 12:00 AM UTC Root cause: A recent change within the Exchange Online service intended to introduce a new virtual account resulted in impact. Next update by: Thursday, March 19, 2026, at 11:30 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps User impact: Users may be intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps. More info: This issue impacts both the Android and iOS mobile app experiences. Current status: We’re continuing our review of Outlook mobile logs and service telemetry to isolate the source of the issue and determine appropriate remediation actions. Scope of impact: Impact may occur for some users intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps. This section may be updated as our investigation progresses. Next update by: Thursday, March 19, 2026, at 6:30 PM UTC

Title: Some users may be intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps User impact: Users may be intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps. More info: This issue impacts both the Android and iOS mobile app experiences. Current status: We're analyzing Outlook mobile logs to isolate the root cause of impact and establish a fix. Scope of impact: Impact may occur for some users intermittently unable to access their Exchange Online mailboxes with the Outlook mobile apps. Next update by: Thursday, March 19, 2026, at 4:30 PM UTC