Sending delayed messages in x-dead-letter-exchange mode with publishing confirmation enabled, delayed messages can be successfully completed. However, every time a delayed message is sent, RabbitTemplate. RabbitTemplate.ReturnsCallback are triggered. Why is this happening?

[nomatterfine]

Describe the bug

Sending delayed messages in x-dead-letter-exchange mode with publishing confirmation enabled, delayed messages can be successfully completed. However, every time a delayed message is sent, RabbitTemplate.ReturnsCallback are triggered. Why is this happening?

RabbitMQ 3.8.35 Erlang 23.3.4.11

Reproduction steps

...
ReturnedMessage [message=(Body:'11' MessageProperties [headers={spring_returned_message_correlation=613060e8-1f2a-4148-96f0-f1f66e7b9340}, contentType=text/plain, contentEncoding=UTF-8, contentLength=0, receivedDeliveryMode=PERSISTENT, priority=0, receivedDelay=2000, deliveryTag=0]), replyCode=312, replyText=NO_ROUTE, exchange=delayed.exchange, routingKey=delayed.routingkey]

Expected behavior

Did I configure it incorrectly or is it that RabbitTemplate.ReturnsCallback are only available in x-dead-letter-exchange mode? If that's the case, I should enable publishing confirmation for delayed messages.

Additional context

No response

[michaelklishin]

RabbitMQ 3.8 is entirely out of support.

The answer to your question is right there in the delayed message exchange README:

Closely related to the above, the mandatory flag is not supported by this exchange: we cannot be sure that at the future publishing point in time

 * there is at least one queue we can route to
 * the original connection is still around to send a `basic.return` to

[michaelklishin]

In other words, since the plugin cannot know if it can send a basic.return in the future, and technically it did not route anything immediately, it sends a basic.return immediately. That does not make much sense but neither does using mandatory publishing with this exchange type.