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A dead letter queue is like a black hole, it will consume the Exchange and the Exchange routing is ended with no indication that it failed.
This works great in the JMS Messaging world where we don't want a bad message to cause endless retries and causing the system to exhaust. The message is said to be poison and thus we want to move it to a dead letter queue so the system can continue to operate and work with the next message.

This default does not go well with other transports using in a request/reply messaging style. If the Exchange failed then the original caller want to be alter it failedwill still want a response after the failure.

So the bottom line is that you must configure and setup the error handling strategies that suits your business needs.