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 operation to identify and isolate IP addresses which are responsible for triggering the third-party anti-spam service to block a portion of Microsoft’s email IP address range, although more time is needed for this to complete. Once complete, we’ll work towards mitigation by limiting the mail flow from these IP addresses.
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: Monday, March 25, 2024, at 11:30 PM UTC
