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 working with the third-party anti-spam service to better identify the IP
address ranges affected by this problem, so that we can more quickly identify and manage sources of potentially malicious email messages. This will help reduce the frequency of spam and phishing email, which would also reduce the number of reported and blocked IP addresses, returning some mail flow as expected. In parallel, we’re reviewing long-term solutions to prevent similar problems.

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: Friday, March 8, 2024, at 9:00 PM UTC