Just setup postfix (and dovecot if you wish to connect with a mail client) on a new droplet, make sure its pointing to something like mail.example.com. Create an A record and call it mail.example.com and have the IP of your mail server be located there. Then create a MX record and have it be mail.example.com.
If you want it to be simple, you could try iRedMail, but I would suggest learning how to set everything up by hand so you could get a good understanding how the software works. It is not hard at all :)