On December 29 2020, some customers provisioned in our US region experienced slowness in outbound message delivery for messages injected using the REST Transmissions API. No messages were lost, although the slowness in delivery occurred over a 7 hour period as we worked to fully restore operations (impact period - 15:16 to 22:15 UTC).

The root cause of this issue was a degradation of software performance on our SMTP outbound delivery tier requiring large scale restarts to restore performance. This degradation caused backups within our mail queues as we worked to restore full service. We are actively working to determine the root cause of the performance degradation with the assistance of our cloud service provider.