Agreed. I use RabbitMQ on my iOS app's server (https://www.letsgohomeapp.com) almost exactly for the OP's purpose; I drop outbound notifications to app users on a RabbitMQ queue and have a separate service which draws from that queue, processes them & sends them off to the correct destination, be it an email address or an iPhone via APNS.

RabbitMQ works great for this & is honestly quite easy to set up and integrate with (was trivial to get up and running in my app's Node.js backend).