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

Timeline

Every update Microsoft posted for this issue, newest first.

1. Nov 2, 2026, 21:09 CET

   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.

2. Oct 27, 2026, 19:10 CET

   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.

3. Oct 22, 2026, 20:34 CEST

   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.

4. Oct 21, 2026, 19:57 CEST

   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.

5. Oct 16, 2026, 20:20 CEST

   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.

6. Oct 14, 2026, 18:14 CEST

   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.

7. Oct 14, 2026, 01:33 CEST

   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.

8. Oct 13, 2026, 23:28 CEST

   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.

9. Oct 13, 2026, 03:12 CEST

   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.

10. Oct 13, 2026, 00:16 CEST

    We're expecting clarity into the timeline for our fix deployment by our next scheduled communication update.

11. Oct 10, 2026, 00:20 CEST

    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.

12. Oct 8, 2026, 23:32 CEST

    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.

13. Oct 8, 2026, 17:20 CEST

    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.

14. Oct 8, 2026, 11:54 CEST

    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.

15. Oct 8, 2026, 01:52 CEST

    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.

16. Oct 7, 2026, 21:30 CEST

    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.

17. Oct 7, 2026, 17:31 CEST

    This investigation is taking longer than expected but continues to yield results that narrow down the root cause and a strategy that remediates impact.

18. Oct 7, 2026, 16:13 CEST

    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.

19. Oct 7, 2026, 14:18 CEST

    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.

20. Oct 7, 2026, 12:29 CEST

    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.

21. Oct 7, 2026, 10:39 CEST

    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.