How to handle unknown/invalid binding routing key values in RabbitMQ?

will remain inside the exchange,

No; exchanges don't "hold" messages, they are simply routers.

Unroutable messages are discarded by default.

You can configure the binding to return unroutable messages.

See Error Channels.

Returns are asynchronous.

In the upcoming 3.1 release, you can wait on a future to determine whether the message was sent successfully or not. See Publisher Confirms.

If the message is unroutable, the correlation data's returnedMessage property is set.

The framework uses the mandatory feature mentioned in another answer.

EDIT

Here is an example:

spring.rabbitmq.publisher-returns: true

spring.cloud.stream.bindings.output.destination: my-exchange
spring.cloud.stream.rabbit.bindings.output.producer.routing-key-expression: headers['rk']

spring.cloud.stream.bindings.output.producer.error-channel-enabled: true

@SpringBootApplication
@EnableBinding(Source.class)
public class So65134452Application {

    public static void main(String[] args) {
        SpringApplication.run(So65134452Application.class, args);
    }

    @Bean
    public ApplicationRunner runner(MessageChannel output) {
        return args -> {
            output.send(MessageBuilder.withPayload("foo")
                    .setHeader("rk", "invalid")
                    .build());
        };
    }

    @Autowired
    RabbitTemplate template;

    @Bean
    public Queue unroutable() {
        return new Queue("unroutable.messages");
    }

    @ServiceActivator(inputChannel = "errorChannel")
    public void error(Message<?> error) {
        if (error.getPayload() instanceof ReturnedAmqpMessageException) {
            this.template.send(unroutable().getName(),
                    ((ReturnedAmqpMessageException) error.getPayload()).getAmqpMessage());
        }
    }

}