From https://issues.apache.org/jira/browse/QPIDJMS-476:

For servers that do not report support for the 'anonymous relay', allowing a single sending link to be created for 'anonymous' producers (either JMSProducer instances, or a MessageProducer created with null as the destination) we implemented a fallback mechanism that created a sender link that is opened on send and closed on completion of the send (remote sends disposition). This mechanism essentially makes every send using the anonymous fallback mechanism synchronous, with the overhead of Attach -> Transfer -> Detach for each call to send, inhibiting performance. We can improve this mechanism and also allow for asynchronous sends both from the JMS 1.1 send methods and from the JMS 2 CompletionListener send methods, as is currently done when the anonymous relay is supported by the server. By allowing for caching of sender links we can also reduce the number of attach and detach cycles for sends that encompass a fixed set of addresses as well.