Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not have been quarantined and failed to be stored in the quarantine folder. More info: The following error appeared in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Final status: We completed deployment of our fix to the affected environments and confirmed with affected users that this action successfully resolved the problem. Scope of impact: This issue impacted users attempting to have email messages quarantined in Exchange Online. Start time: Wednesday, September 23, 2026, at 12:56 AM UTC End time: Friday, October 2, 2026, at 8:24 AM UTC Root cause: A recent service-side update introduced a code error that caused this impacting event. Next steps: We're reviewing the service-side update that introduced the code error to understand why it resulted in this impact and help prevent similar issues from occurring in the future. This is the final update for the event.

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: The following error appears in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Current status: We're continuing to monitor the deployment of our fix, which has reached 88 percent saturation to the affected environment. We anticipate the deployment will complete by our next scheduled communications update. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Start time: Wednesday, September 23, 2026, at 12:56 AM UTC Estimated time to resolve: We anticipate the fix will complete deployment by Friday, October 2, 2026, at 7:00 PM UTC. Root cause: A recent service-side update introduced a code error that has caused this impacting event. Next update by: Friday, October 2, 2026, at 7:00 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: The following error appears in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Current status: We've initiated the fix release and are monitoring as it progresses. We expect to have a completion timeline by the next scheduled update. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Root cause: A recent service-side update introduced a code error that has caused this impacting event. Next update by: Thursday, October 1, 2026, at 11:30 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: The following error appears in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Current status: Validation of our fix is in its final stages, and we expect to begin deploying it to the affected environment by our next scheduled update. We’ll provide an estimated completion timeline once available. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Root cause: A recent service-side updated unintentionally introduced a code error that has caused this impacting event. Next update by: Thursday, October 1, 2026, at 6:00 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: The following error appears in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Current status: Validation of our fix is in its final stages, and we expect to begin deploying it to the affected environment by our next scheduled update. We’ll provide an estimated completion timeline once available. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Root cause: A recent service-side updated unintentionally introduced a code error that has caused this impacting event. Next update by: Thursday, October 1, 2026, at 6:00 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: The following error appears in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Current status: We've identified a recent service-side update that introduced an impacting code error that's causing some email messages to not be quarantined and fail to be stored in the quarantine folder. As we develop a code fix to remediate the impact, we're exploring a secondary potential mitigation option involving moving affected service traffic. We're expecting more clarity into our mitigation action plan by our next scheduled communication update. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Root cause: A recent service-side updated unintentionally introduced a code error that has caused this impacting event. Next update by: Thursday, October 1, 2026, at 1:00 AM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: Affected emails cannot be released through the standard Microsoft Defender quarantine workflow. The following error appears in message trace: '‎550 5.1.10 Failed to deliver to Quarantine store‎'. Current status: A recent service-side update unintentionally introduced a code error that is causing legitimate email messages to be incorrectly quarantined and not delivered to recipients. We're developing a code fix and actively validating and testing the remediation in our test environment before broader deployment. We expect this validation work to be completed before our next scheduled communication update. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Start time: Wednesday, September 23, 2026, at 12:56 AM UTC Root cause: A recent service-side updated unintentionally introduced a code error that has caused this impacting event. Next update by: Thursday, October 1, 2026, at 11:00 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: Affected email can’t be released through the normal Microsoft Defender quarantine workflow. Current status: We've received the requested network message IDs (NMIDs) for failing email deliveries and are analyzing them in order to assist with our ongoing root cause investigation. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Next update by: Thursday, September 24, 2026, at 11:00 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: Affected email can’t be released through the normal Microsoft Defender quarantine workflow. Current status: We're analyzing potential email processing failures from the provided message IDs. We're working with some reporting users to collect and provide network message IDs (NMIDs) from failing email deliveries to assist with our troubleshooting analysis. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Next update by: Thursday, September 24, 2026, at 8:00 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: Some legitimate email messages are not being delivered to the intended recipient and cannot be released through the normal Microsoft Defender quarantine workflow. Current status: We're reviewing message IDs to help determine the next steps to reach a mitigation plan and root cause analysis. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Next update by: Thursday, September 24, 2026, at 4:30 PM UTC

Title: Some users’ email messages may not be quarantined and fail to be stored in the quarantine folder User impact: Users’ email messages may not be quarantined and might fail to be stored in the quarantine folder. More info: Some legitimate email messages are not being delivered to the intended recipient and cannot be released through the normal Microsoft Defender quarantine workflow. Current status: We're reviewing support case details to determine our next troubleshooting steps. Scope of impact: This issue may impact users attempting to have email messages quarantined in Exchange Online. This section may be updated as the investigation progresses. Next update by: Thursday, September 24, 2026, at 1:30 PM UTC