Some users’ outbound Exchange Online email messages may be marked as spam and not delivered

Title: Some users’ outbound Exchange Online email messages may be marked as spam and not delivered

User impact: Users’ outbound Exchange Online email messages may be marked as spam and not delivered.

More info: This isn’t connection method specific and thus occurs in all Exchange Online connection methods. Affected users receive a Non-Delivery Report (NDR) message that references the third-party anti-spam service name that has added the IP address to their block list.

Current status: Our service health telemetry indicates that the previously mentioned service optimizations have had the desired effect, and we’ve observed a significant reduction in the volume of impacted messages. We’re continuing to implement further optimizations to reduce or eliminate impact as quickly as possible, and in parallel we’re continuing our efforts in coordination with the third-party anti-spam service to identify and implement a long-term solution to fully remediate the issue.

Scope of impact: The problem may impact some users sending outbound email messages if they’re leveraging a specific third-party anti-spam service mentioned within the NDR.

Start time: Thursday, April 11, 2024, at 6:30 PM UTC

Root cause: A third-party anti-spam service is blocking a portion of Microsoft’s email IP address ranges to protect organizations that use their services.

Next update by: Wednesday, April 24, 2024, at 11:00 PM UTC