From https://issues.apache.org/jira/browse/QPID-3503:

One of our clients is interested in logging information about any message that the broker discards because the message expired (ttl). Because our client's logging requirements are specific to the system we're building for them, rather than trying to modify the log message the broker generates when it discards an expired message, we've created an option that can be set on a queue called qpid.expiry_exchange. If set, whenever the broker discards an expired message, instead of just deleting it, it will send it to the specified expiry exchange. This way, we can bind a queue to the expiry exchange and attach a separate Receiver to the queue to receive these expired messages and log them.