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 the process of identifying the IP addresses that are causing the third-party anti-spam service to activate and block a segment of Microsoft’s email IP address ranges. While we are seeing signs of improvements to some IP address ranges that we’ve identified thus far, we’ll share an updated timeline for this investigation and the steps being taken to remediate the impact when more information is available.
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, April 3, 2024, at 11:00 PM UTC
