The dead letter queue is a queue in which Broker places dead letters. A dead letter is a document for which the Broker has no subscribers. For example, if a client program publishes a document of type "Orders::ShippingNotice," and the Broker has no subscribers for that document type (or if it has filtered subscriptions for the document type, but the document instance does not match the conditions specified by the filters) it places the document in the dead letter queue. A Broker also sends to the dead letter queue delivered documents that are addressed to non-existent clients.