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.”
Current status: We’re continuing our collaboration with the third-party anti-spam service to delist affected IP addresses that are being blocked. In parallel, we’re investigating any additional workstreams that can further reduce the impact of this issue. This may include further rule changes intended to reduce outbound spam patterns. We understand the impact an issue like this can have on your organization, and we appreciate your partnership and patience as we work to remediate the issue.
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: Friday, January 26, 2024, at 9:30 PM UTC