humans only junk email filtering

This is an idea for blocking junk email. You know how some free email providers like yahoo etc have a new account sign up system where you have to prove you are a living breathing human with a brain by correctly answering a random question. They do it by severely distorting a single word in a graphic. If you have a human brain, you get the right answer and you are given a new email account. But that method could also be used in the reception of all email. A new class of email protocol could be invented and the current POP email systems could be abandoned in several years. Lets say the new email system becomes called POPproof accounts. And lets say POPproof email address have an extra @ symbol to distinguish them from the old email method. So you are a human, and you want to send email a friend or a stranger. Their email address is sam@@superhost.com. You click send. But a box pops up on your screen. It says, "The receiving email server wants you to prove you are a real living person" "It wants to know if you can see the distorted word in this image". Luckily because you have a human brain you see it, and you type the word that you see and your email is successfully sent.