Microsoft 365 Alert – Service Degradation – Exchange Online – Some users may be unable to send or receive email messages and receive a Non-Delivery Report (NDR) – RESOLVED

05/01/2024 08:53:00 AM

NHSMail Reference: INC40696993

Microsoft Reference: EX703958

Issue Status: RESOLVED

Issue Description: Users may be unable to send or receive email messages and receive a NDR with a 550 or 554 error code.

More info: This issue only occurs if either the sender or user who is receiving the message is utilizing a specific third-party anti-spam service. Senders receive the NDR which states that the message has been rejected due to a 550 or 554 error code and that their host IP address is listed on the third-party anti-spam service.

The error messages state “550 5.7.350 Remote server returned message detected as spam”, “554 5.7.1 Service unavailable; Client host [] blocked”, or “‎550 5.7.1 This email was rejected because it violates our security policy.”

Final Update: 06/02/2024 14:26:00 PM Microsoft confirmed after extensive monitoring that their 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 affect any user’s mail flow if they are leveraging a specific third-party anti-spam service to filter email messages.

Root cause: Outbound spam associated with some Microsoft IP addresses is causing all mail from these addresses to be blocked by a specific third-party antispam service.

Next steps: Microsoft are exploring monitoring optimizations so they can more quickly identify and work to delist blocked IPs should similar future events occur.

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