"AdrianWoodhead" wrote : The clients are calling send() on the same MessageProducer instance, and this isn't synchronized. This hasn't caused any noticeable problems in the past (when the MessageProducer was using ActiveMQ) or since we started evaluating JBM (although the loads on those 2 servers are still very low). This makes me think I really should queue up the messages to be sent myself and have one thread call send(), which will also get rid of the blocking problem.. +1 You may have just got lucky in the past. All sorts of weird subtle errors might start happening if you use a JMS session concurrently with different threads. http://java.sun.com/javaee/5/docs/api/javax/jms/Session.html "...A Session object is a single-threaded context for producing and consuming messages..." View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4242377#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...