Emails, domain names and spamming

I recently acquired a domain name for a registrar. I later on proceeded in buying some hosting space in order to create email mailboxes for my company. During the registration process for the hosting space, I entered my domain. I successfully created my first mailbox and sent a test email. I received the email immediately.

Nobody checked somehow if the domain I entered was indeed mine, and this put me into thinking if someone could do the same and put e.g paypal.com as his domain name and starts sending emails pretending to be someone else. How is the Inteet community protected from this kind of things?