Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may have received duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' may have been delivered more than once to external recipients. This may have resulted in duplicate tickets, updates, workflows, or notifications in downstream systems. Final status: We've validated that our deployed fix finished saturating throughout the affected environment and has successfully remediated all impact for this issue. Scope of impact: Any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding may have been impacted by this event. Start time: Wednesday, September 30, 2026, at 9:36 PM UTC End time: Wednesday, November 11, 2026, at 4:14 PM UTC Root cause: A recent service update caused abnormal headers to be generated during the forwarding process, which resulted in the duplicate email messages being created. Next steps: - We're reviewing our service update check and implementation processes to improve how we detect potential Exchange Online SMTP forwarding header issues and prevent similar service events from occurring in the future. This is the final update for the event.

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' may be delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: Our deployed solution to the affected environment has completed approximately 96 percent of its deployment, and we're projecting it will have completed and for the impact will be remediated by our next scheduled communication update. Scope of impact: Any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding may be impacted. This information may be updated as our investigation continues. Start time: Wednesday, September 30, 2026, at 9:36 PM UTC Estimated time to resolve: Our fix deployment timeline expects the deployment will have completed and that the impact will be remediated by Wednesday, November 11, 2026. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Wednesday, November 11, 2026, at 8:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' may be delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We've validated our fix's efficacy in remediating the impact, and we're preparing to deploy the fix to the remaining affected environment. We'll aim to provide a timeline for the deployment to complete by our next scheduled communications update. Scope of impact: Any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding may be impacted. This information may be updated as our investigation continues. Start time: Wednesday, September 30, 2026, at 9:36 PM UTC Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Monday, November 9, 2026, at 8:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' may be delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We've completed our deployment of the fix to our internal test environments and finished validating the fix. We've progressed now to expanding the fix deployment to all affected environments. We anticipate having a better timeline for the deployment completion by our next communication time. Scope of impact: Any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding may be impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Thursday, November 5, 2026, at 8:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' may be delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're deploying the fix to our internal test environment to conduct the final validations prior to beginning the broad deployment. We anticipate the validations will be complete by our next scheduled communications update. Scope of impact: Any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding may be impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Monday, November 2, 2026, at 8:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' may be delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're continuing to validate the efficacy of our fix to correct the abnormal headers and will provide a timeline for the deployment as one becomes available. Scope of impact: Any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding may be impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Tuesday, October 27, 2026, at 7:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We've completed the development of a fix and are in the process of validation. We'll provide a release and mitigation timeline once available. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Thursday, October 22, 2026, at 7:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We've determined that an additional fix is required to fully correct the impacting header behavior, and we're continuing to develop this fix while, in parallel, we're exploring our options to expedite the deployment once the fix has been validated. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Wednesday, October 21, 2026, at 6:00 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: Our investigation into what's causing the impact to persist is ongoing, and we're determining whether our original fix didn't completely address the issue with the abnormal headers. In parallel, we're preparing to develop another fix to correct any remaining issues unaddressed by our original fix. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Friday, October 16, 2026, at 6:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We’re investigating reports from some affected users indicating duplicate messages are still being received. We’re reviewing service telemetry to determine why the impact appears to be persisting as our fix saturates and whether additional remediation is required. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Wednesday, October 14, 2026, at 5:00 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: While the fix continues to saturate, we've received reports from a subset of affected users that the impact persists. We're continuing to monitor the saturation while we validate that it's remediating the impact as expected. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Tuesday, October 13, 2026, at 11:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: Our fix deployment is nearing completion, and we suspect that it has reached most affected users. We’re continuing to monitor the deployment and will provide an estimated completion time once available. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Tuesday, October 13, 2026, at 9:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're expecting clarity into the timeline for our fix deployment by our next scheduled communication update. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Tuesday, October 13, 2026, at 2:30 AM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: The deployment of the fix is progressing more slowly than anticipated to your users. We'll continue monitoring to ensure its success and provide a timeline to completion as one becomes available. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Monday, October 12, 2026, at 10:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We've confirmed that the aforementioned service update is causing the impact. We're deploying a fix to correct the headers and remediate the impact. We'll provide a timeline for the completion of the deployment as one becomes available. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Root cause: A recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. Next update by: Friday, October 9, 2026, at 10:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: From our review of the reproduction, we suspect that a recent service update is causing abnormal headers to be generated during the forwarding process, resulting in the duplicate email messages being created. We're continuing to review the reproduction to validate our root cause understanding, while we develop a fix to resolve the issue. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Thursday, October 8, 2026, at 11:00 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We've obtained a reproduction from a subset of affected users following the implementation of a configuration change. We’re comparing affected and unaffected message samples to determine whether Active Directory data or operations are contributing to the abnormal duplicate header values. This analysis will help us further isolate the underlying cause and guide our next troubleshooting steps to reach a mitigation plan. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Thursday, October 8, 2026, at 4:30 PM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We’re continuing our analysis of the affected messages and have asked a subset if affected users to reproduce the impact following the implementation of a configuration change. The results may provide further insight into the underlying cause and guide our next troubleshooting steps. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Thursday, October 8, 2026, at 10:30 AM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: Our analysis has ruled out multiple potential causes, but due to the complex nature of this issue, we haven't isolated the source of this issue yet. We're continuing our investigation to identify and analyze all possible contributors to this impact. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Thursday, October 8, 2026, at 1:30 AM UTC

Title: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: This investigation is taking longer than expected but continues to yield results that narrow down the root cause and a strategy that remediates impact. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 8:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're continuing to investigate why some messages are being delivered more than once while others using the same forwarding setup aren't affected. We've confirmed the behavior isn't tied to a specific recipient and are now focusing on what may have changed when the issue first started appearing so we can better understand why the duplicate deliveries occur. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 4:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're working to determine why duplicate deliveries occur for some messages while other messages using the same forwarding configuration are delivered only once. We've confirmed differences in how affected and unaffected messages are being processed and are continuing to review support-provided examples to better understand what conditions lead to the duplicate deliveries. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 2:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're reviewing additional message examples to better understand why duplicate deliveries are occurring in some cases but not others when the same forwarding method is used. This is helping us identify the specific circumstances that lead to duplicate messages being delivered so we can determine the most appropriate path forward. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 12:30 PM UTC

Title: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding User impact: Users may receive duplicate email messages sent to mailboxes using Exchange Online SMTP forwarding. More info: Messages sent to mailboxes configured with Exchange Online SMTP forwarding and 'DeliverAndForward' are being delivered more than once to external recipients. This may result in duplicate tickets, updates, workflows, or notifications in downstream systems. Current status: We're reviewing support-provided information and comparing examples of duplicated and non-duplicated messages to better understand why this behavior occurs intermittently. This is helping us determine the conditions under which duplicate message delivery can occur and identify appropriate next troubleshooting steps. Scope of impact: Your organization is affected by this event, and any user expecting email messages in an Exchange Online mailbox configured with SMTP forwarding is impacted. This information may be updated as our investigation continues. Next update by: Wednesday, October 7, 2026, at 10:30 AM UTC