@guinslym for the sake of internet as whole, and your own, do not try to set up an email server. It's really a bad idea unless you really know what you're doing and you're willing to do it on regular basis. I was always recommended not to maintain an email server on my own, so I'm passing on that recommendation.
Instead, you could keep using shared hosting service, and set up your droplet's DNS records so it keeps using your shared hosting for email. I just opened up a question regarding that particular request, you can keep track if you want.