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?
Why are you considering RabbitMQ ? it is better to consider using a MTA/Mail relay like Postfix where you submit your emails and it handles them for you in a queue.
You can configure it to dispatch the queue on different mail relays, set the email throughput, how much retry shall be made on a failed sending ...
At what email volume to use AMQP