A message queue is nothing more than a sequenced list of messages. Store-and-forward message switches receive messages from a variety of communication lines over a variety of protocols, convert them to a standard format, and save them in message queues until they are requested by the destination nodes in the network. Their queues are prioritized and persistent. Queue management is usually under operator control from a console.
TP systems require that a message is queued in order to serialize requests. They also require that the request message will be restored to the queue if the transaction aborts or the system fails.