The dead-letter queue (or undelivered-message queue) is the queue to which messages are sent if they cannot be routed to their correct destination.
Messages are put on this queue when they cannot be put on the destination queue. For example, because the queue does not exist, or because it is full. Dead-letter queues are also used at the sending end of a channel, for data-conversion errors.
Consider defining a dead-letter queue for each queue manager. If you do not, and the MCA is unable to put a message, it is left on the transmission queue and the channel is stopped.
Also, if fast, non-persistent messages (see Fast, nonpersistent messages ) cannot be delivered, and no dead-letter queue exists on the target system, these messages are discarded.
However, using dead-letter queues can affect the sequence in which messages are delivered, and so you might choose not to use them.
You can use the USEDLQ channel attribute to determine whether the dead-letter queue is used when messages cannot be delivered. This attribute can be configured so that some functions of the queue manager use the dead-letter queue, while other functions do not. For more information about the use of the USEDLQ channel attribute on different MQSC commands, see DEFINE CHANNEL, DISPLAY CHANNEL, ALTER CHANNEL, and DISPLAY CLUSQMGR.