A message can be either durable or non-durable. This shouldn't be confused with whether the delivery will be reliable or not. Although reliable delivery does require durability, a durable message can be sent over a non-reliable transport. In other words, while the transport may promise to do it's best to hold onto the message for its intended recipient, it may not promise utter reliability, or may allow the recipient to receive the same message twice, or it may allow messages to be received out of order.