The message headers provide metadata describing who or what created the message, when it was created, how long its data is valid, etc. The headers also contain routing information that describes the destination of the message (topic or queue), how a message should be acknowledged, and a lot more.

The Message interface provides mutator ("set") methods for each of the JMS headers, but only the JMSReplyTo, JMSCorrelationID, and JMSType headers can be modified using these methods. Calls to the mutator methods for any of the other JMS headers will be ignored when the message is sent. According to the authors of the specification, the mutator methods were left in the Message interface for "general orthogonality"; to balance the accessor methods—a fairly strange but well-established justification.

The accessor ("get") methods always provide the JMS client with information about the JMS headers. However, some JMS headers (i.e., JMSTimestamp, JMSRedelivered, etc.) are not available until after the message is sent or even received.