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 to work with the third-party anti-spam service to monitor the IP address ranges affected by this problem and identify which may be sources of potentially malicious email messages so we can determine our next troubleshooting steps for reducing such email messages and the frequency of those IP addresses being blocked. In parallel, we’re continuing to assess potential long-term solutions to address this and similar problems in the future.

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 12, 2024, at 11:00 PM UTC