Update: Apple has marked the issue as resolved and we have seen a significant decrease in the rate at which inaccurate 550 SMTP responses are being issued to message delivery attempts. All messages previously held in queue due to these errors have been successfully delivered.
Apple has reported a service issue for users of iCloud Mail. Only some users are affected. Users may be experiencing intermittent issues with this service. You can follow the status of this issue with Apple on their status page here: https://www.apple.com/support/systemstatus/
SocketLabs has identified Apple iCloud is improperly returning invalid address errors such as "550 5.1.1 : user lookup success but no user record found" for the impacted users. We have modified system behavior to treat this error as a temporary issue. Messages encountering this error response will be held in queue and automatically retried for delivery at an optimized frequency. As a result of this issue at Apple some SocketLabs customers may be seeing a higher than normal volume of messages being held in queue to domains operated by Apple.
Additionally, SocketLabs has proactively removed all addresses added to customer suppression lists today due to this error response. Further questions regarding this issue can be directed to [email protected]