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: Our efforts to collaboratively delist affected IPs with the third-party antispam service are continuing on a case-by-case basis as we identify additional blocked addresses. In parallel, we’re examining IP block patterns we’ve identified via our telemetry so we can pinpoint the most efficacious rule changes and optimizations to reduce outbound spam and alleviate impact.
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.
Next update by: Tuesday, January 30, 2024, at 12:00 AM UTC
