title: How to properly implement an email tracking pixel
body: <p>I am current using PHP to output a 1x1 pixel that I include in e-mails.
  Before I output the image, I run a couple of scripts (i.e. increment a view count etc...).
  However, I've noticed that clients like GMail &amp; Outlook download the image before serving it to the user and, of course, this counts as a view, because the image is being viewed. I send the e-mail and before even opening it I get a response in my server that the tracking pixel has been viewed and then when I open the e-mail, I get a second response.</p>
  
  <p>My question is, within the tracking pixel, how can I truly tell when the user has opened the e-mail and not when a client like GMail or Outlook is just downloading the image?</p>