Actually, what JBoss is doing is copying the properties from the original message into a "cloned" message. The JMSXDeliveryCount is a property coming from the original "IBM" message. Apparently, IBM makes it read-only, or only available by the receiver.
From the JavaDoc:
The JMS API reserves the JMSX property name prefix for JMS defined properties. The full set of these properties is defined in the Java Message Service specification. New JMS defined properties may be added in later versions of the JMS API. Support for these properties is optional. The String ConnectionMetaData.getJMSXPropertyNames method returns the names of the JMSX properties supported by a connection.
JMSX properties may be referenced in message selectors whether or not they are supported by a connection. If they are not present in a message, they are treated like any other absent property.
JMSX properties defined in the specification as "set by provider on send" are available to both the producer and the consumers of the message. JMSX properties defined in the specification as "set by provider on receive" are available only to the consumers.
JMSXGroupID and JMSXGroupSeq are standard properties that clients should use if they want to group messages. All providers must support them. Unless specifically noted, the values and semantics of the JMSX properties are undefined.