Title: Some users may be unable to send or receive email messages, and receive a Non-Delivery Report (NDR) when sending
User impact: Users may be unable to send or receive email messages, and receive an NDR with a 550 or 554 error code when sending.
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 [xxx.xxx.xxx.xxx] blocked”, or “550 5.7.1 This email was rejected because it violates our security policy.”
Current status: We’re continuing to see positive trends in our monitoring telemetry, and we’re confident that a large percentage of users are no longer affected by this issue. As our mitigative changes take effect, we recommend that users retry sending email messages that previously returned NDRs, as this may now prove successful. In parallel, we’re performing some additional rule optimizations to remediate impact for any users who may be residually affected by this problem.
Scope of impact: This issue may affect any user’s mail flow if the recipient is leveraging a specific third-party anti-spam service to filter email messages.
Start time: Thursday, December 25, 2025, at 12:00 AM UTC
Preliminary 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 update by: Friday, February 2, 2024, at 12:00 AM UTC
