I'm new to this and I've read a tutorial on this page: http://www.melyssagriffin.com/use-gmail-custom-domain/
The author of the tutorial wrote about her experiences of setting up a custom domain name in her e-mail address for her webpage so something like email@example.com. Now, what I don't understand is how the confirmation process works i.e. how the email provider verifies that I really own the domain. And this seemed weird to me - Gmail only requires that the owner of the website puts some unique HTML code on their website as a confirmation. What surprised me is that there was no third-party involved. Nobody contacted for example the company who registered the domain (e.g. GoDaddy) or anyone else. It seems to me that a Gmail bot just scanned the webpage, saw the pasted HTML code and basically said "ok, you're good to go".
So what does prevent me from becoming a new email provider and setting up an email with an already taken domain like firstname.lastname@example.org or email@example.com?