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: We’re continuing our efforts of working with the third-party anti-spam service to monitor the IP address ranges affected by this problem and identify which IP addresses may be causing the mass delivery of malicious email messages. Once we’ve isolated the IP addresses that are causing the third-party anti-spam service to block a portion of Microsoft’s email IP address ranges, we’ll limit the mail flow as a long-term solution to prevent the issue from reoccurring.

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: Tuesday, March 19, 2024, at 11:00 PM UTC