DigitalOcean provides un-managed hosting. That means you get the power of full root access to your VPS, but it also means you need to set things up your self. There are a number of services that can provide email for you domain, such as Google Apps. You might want to check with your domain registrar. They often provide free email forwarding for domains. If you want to set up a full blown email server, you might want to check out iRedMail:
If you would like to create your own online e-mail system, you can use iRedMail. In this article we will explain how you can do it on your Ubuntu 12.04 virtual server.