Ownership over the email shortname is exactly what is being proven. It’s your identity as far as the service is concerned. I can block emails from a service right after verifying so verifying does not guarantee that I will receive emails from the service at any point in the future.
If you do block it, your access to the account you’ve created might be in jeopardy. At least it depends on how you “block” it. If the service receives an error on future emails, the user’s account can be disabled, as usually described in the TOS.
