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’ve performed some rule optimizations to minimize outbound spam and, while we’re continuing to monitor for new reports of impact, our signals indicate a decrease in addresses blocked by the third-party antispam service. We’ve also confirmed that many formerly blocked IP addresses have since been delisted automatically, providing relief for many users. We’ll continue monitoring telemetry for an extended period to ensure that this positive trend persists before confirming resolution.
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: Wednesday, January 31, 2024, at 12:00 AM UTC
