You should already have a worker that can send the email, just now you need setup RabbitMQ with priorities.
The main exchange you use,
email-undeliverable that is declared as a fanout exchange. Now you just
default queue bound to the
default routing key for the
email-undeliverable. Now every email your try
to send that doesn't have a specifically prioritized queue will be routed
All you need now is to start your workers consuming from each queue you create.