Some users may be unable to send or receive email messages, and receive a Non-Delivery Report (NDR) when sending

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.”
We recommend that users retry sending email messages that previously returned NDRs, as this may now prove successful.
Current status: Our monitoring telemetry continues to indicate a positive trajectory, signaling relief for many users. As previously mentioned, efforts are ongoing to explore additional rule optimizations so we can ensure that impact for residually affected users is effectively minimized.
Scope of impact: This issue may affect any user’s mail flow if the recipient is leveraging a specific third-party antispam 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: Tuesday, February 6, 2024, at 12:00 AM UTC