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

Title: Some users may be unable to send or receive email messages and receive a Non-Delivery Report (NDR)
User impact: 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 [xxx.xxx.xxx.xxx] blocked”, or “‎550 5.7.1 This email was rejected because it violates our security policy.”
Current status: We continue to collaborate with the third-party service to delist additional IP addresses that are being blocked on the third-party’s anti-spam service. In parallel, we continue to analyze outbound spam patterns and implement targeted rules and optimizations for these patterns, which has produced a reduction in the number of blocked instances.
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: Monday, January 22, 2024, at 7:30 PM UTC