title: How to know if the user input a registered valid email address that exists?
body: <p>I want to do is make a registration form with a email required but the user should input a valid registered email address. If the email is not valid, then the registration is not complete. How do I do that?</p>
  
  <p>Example:</p>
  
  <pre><code>&lt;form&gt;
      &lt;input type="email" name="email" required="required" /&gt;
      &lt;input type="submit" value="submit" name="submit" /&gt;
  &lt;/form&gt;
  </code></pre>