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 previously mentioned operation to isolate the IP addresses that are causing the third-party anti-spam service to block a portion of Microsoft’s email IP address ranges, after which we’ll proceed with limiting the mail flow 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: Thursday, March 21, 2024, at 11:30 PM UTC
