To my knowledge the whole "unregistered" thing mainly occurs with the free webmail services. This is because they are free, and therefore the mail provider has to regularly purge inactive accounts to save space on their server. That's why once registered emails become unregistered. However, I think with a private service they may not purge them at all and may not be able to recreate unless you are a subscriber. This is true with certain internet providers like verizon and charter. You can only access or create a mail account with them if you are a customer, and you have to prove you are.
This is mainly speculation, but hopefully that helps.
If you want a specific domain name, you must contact the website administrator, otherwise the email when registered will say (unregistered) rendering it useless.
If you actually own a domain name, you can make your own e-mail addresses using that domain address.