Dead Letter Queue (DLQ) is a destination for messages that the server has failed to deliver more than a certain number of times. If DLQ is not specified, the message will be removed after the maximum number of delivery attempts. This will help you identify the message delivery exceptions and ensure that the messages are not lost. You can read the messages from DLQ and fix the application accordingly.
By default, DLQ is configured in JBoss Messaging at the global level for all queues.