I have an application that emails individuals on different occurrences. The entire application is on a single server. I am currently sending emails through SendGrid. At what volume of emails would it make sense to use a system like RabbitMQ to send out emails? 
Maximum rate = 1 email per minute? 1 email per second? 10 emails per second? How would I evaluate when the switch makes sense?
Having Rabbitmq is good option when your are considering scaling in future, I mean in terms of new smpt send mail workers or new email server, as of now if you have single server and not going to more in it then rabbitmq will load your server even more and will be issue to maintain, but if your going to have more then 100 mails per second then it makes sense to have rabbitmq its goal to make your calling function free as soon as possible by offloading all the load from function to rabbitmq queue and then save it till worker or consumer doesn't pick them, this will help in fail cases too as your having your mails saved in rabbitmq and if the consumer fails you still have your mails, when it starts (smtp send worker) then rabbitmq will provide the rest of the mails to it. I hope this makes sense, please feel free to ask other stuff about it, I used rabbitmq for sending mail but in my case we are having one server running only rabbitmq only so there it makes sense.
At what email volume to use AMQP