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 mitigation efforts are underway as we continue to work with the third-party anti-spam service to monitor the relevant IP ranges and isolate which IP addresses may be responsible for the malicious email messages causing the service to block Microsoft IP ranges. We’ll be able to limit the mail flow from these once identified, which should serve as a long-term solution for this 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.
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, March 20, 2024, at 11:30 PM UTC
