After you move a few tasks to the background with RabbitMQ you may realize that you eventually need to support different priorities for the same type of tasks, sending bulk email after you send transactional email. RabbitMQ doesn't have priorities so you wind up having to use separate queues for each priority.

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, should be declared either topic or direct and will take all of the messages you intend to send but when declared you should include an alternate exchange of email-undeliverable that is declared as a fanout exchange. Now you just need a default queue bound to the default routing key for the email exchange and also bound to email-undeliverable. Now every email your try to send that doesn't have a specifically prioritized queue will be routed to the default queue.

All you need now is to start your workers consuming from each queue you create.