title: Is it a good practise to validate email addresses using SMTP validation?
body: <p>Let's imagine that I have a site with some features that require email validation (eg. user registration). Of course I will validate emails using regex but once upon a time I saw SMTP validation in someone else's code. What good and bad sides has SMTP validation?</p>
  
  <p>I can assume the following pros and cons:</p>
  
  <p><strong>Pros:</strong></p>
  
  <ul>
  <li>We can check out some email addresses for existence (not all SMTP servers allows this feature).</li>
  </ul>
  
  <p><strong>Cons:</strong></p>
  
  <ul>
  <li>Our site can be <a href="http://www.spamresource.com/2007/01/whatever-happened-to-vrfy.html" rel="nofollow">blocked</a>.</li>
  <li>Maybe we can use this to make webserver going down. Webserver will take domain part of email address and make request to it. We can create fake SMTP server that will receive request but will make great delay to response. Maybe if we force webserver to make dozens of such checks it will not be able to answer other clients.</li>
  <li>Not all SMTP servers provide this capability.</li>
  </ul>
  
  <p>Please be constructive.</p>