Title: Some users may be unable to send or receive email messages, and receive a Non-Delivery Report (NDR) when sending
User impact: Users were unable to send or receive email messages, and received an NDR with a 550 or 554 error code when sending.
More info: This issue only occurred if either the sender or user who was receiving the message was utilizing a specific third-party anti-spam service. Senders received the NDR which stated that the message was rejected due to a 550 or 554 error code and that their host IP address was listed on the third-party anti-spam service.
The error messages stated “550 5.7.350 Remote server returned message detected as spam”, “554 5.7.1 Service unavailable; Client host [xxx.xxx.xxx.xxx] blocked”, or “550 5.7.1 This email was rejected because it violates our security policy.”
We recommend that users retry sending email messages that previously returned NDRs, as this should now prove successful.
Final status: We’ve confirmed after extensive monitoring that our work to unblock affected IP addresses has successfully reduced the rate of impacted mail flow to pre-incident thresholds, mitigating impact for users.
Scope of impact: This issue may have affected any user’s mail flow if the recipient was leveraging a specific third-party antispam service to filter email messages.
Start time: Thursday, December 25, 2023, at 12:00 AM UTC
End time: Monday, February 5, 2024, at 10:13 PM UTC
Preliminary root cause: Outbound spam associated with some Microsoft IP addresses was causing all mail from these addresses to be blocked by a specific third-party antispam service.
Next steps:
– We’re exploring monitoring optimizations so we can more quickly identify and work to delist blocked IPs should similar future events occur.
