How to change a droplet name, and all associated configurations?

I’ve seen other responses to this subject, but I’m still not sure. I bought my droplet with a friend, and since both of us have websites, we didn’t use a droplet/host name of either. we just used a random name (without any domain) for it.

But we are using this to host our websites and applications. Given that, we need email forwarding and SMTP capabilities (not necessarily a full blown SMTP server, but at least a way to forward emails to any domain pointing at the server, and ability to send emails through php’s mail() function without hassle).

I’ve seen tutorials for this and most of them require the droplet/host name’s to be a valid domain name.

Is there any other way to do it? If not, what else do I need to change, appart from renaming the droplet and changing the hostname of the server? Is it that simple? :)

I’m using Ubuntu 14.04.