Setting up a hostname for mx record

I am having problems with setting up a mail server on my droplet. here are my confusions

1. When adding an mx record, I am asked to add something like mail.sitename.com, my question is how do I setup this sub-domain or is it set setup automatically?

2. I have already installed postfix and dovecot but I cant receive any emails sent to the created address during configuration

3. How do i install roundcube to my droplet, I have seen varying tutorials on this and its getting confusing

I am running Ubuntu 14.04 on my digitalocean.

Thanks.