The AWS SQS backend behind the MirrorQueue handle, mapping its receive-process-ack shape onto SendMessage, ReceiveMessage, DeleteMessage, and ChangeMessageVisibility. Retry is don't ack: SQS redelivers an unacked message, and deadLetter returns a terminal one under a backoff longer than the processing window without deleting it, so it rides the operator's redrive policy to the dead-letter queue. Every failure is a TransportFault value, never an exception. The queue is an operator-declared destination, so the data-plane egress controls of Ecluse.Core.Security.Egress do not apply to it.