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.

Final status: We’ve seen drastic improvements and significant reduction in the volume of impacted messages due to our implemented optimizations. We continue to proactively identify and stop spam messages through our ML antispam systems, as well as respond and implement further adjustments manually to block messages we manually find. Given that our optimizations have been successful, and because there’s no means to prevent all spam messages from ever occurring, we’re declaring this event resolved.

Scope of impact: The problem may have impacted some users that sent outbound email messages if they were leveraging a specific third-party anti-spam service mentioned within the NDR.

Start time: Tuesday, January 2, 2024, 12:00 AM UTC

End time: Wednesday, April 24, 2024, at 10:30 PM UTC

Root cause: A third-party anti-spam service was blocking a portion of Microsoft’s email IP address ranges to protect organizations that used their services.

Next steps:
– We’re further working alongside the third-party anti-spam service to determine a proactive measure to identify Microsoft’s email IP address that are causing future impact and to implement optimizations before impact reoccurs on a broader scale.

This is the final update for the event.