The Certificate Authority must simply verify that the person or organization applying for the certificate owns the registered domain. To satisfy the Domain Validation requirement you must prove you own the domain that was submitted with the order.
The easiest, and most preferred method for accomplishing this is via email-based authentication. During email-based authentication, the CA will send an email to the WHOIS registrar email address asking them to verify that they did indeed register for a certificate. Once you respond to that email in the affirmative, the requirement is considered satisfied and the certificate is issued.
This can be done in just minutes.
The CA can also send the authentication email to one of five pre-approved email addresses associated with the website. The five pre-approved addresses are: