title: How to know that an email message was read?
body: <p>I have a software that sends notifications, quotes and invoices to "clients of my clients" by email. Sometimes people don't answer it very fast, so someone needs to call by phone to confirm if they received and get the feedback. I would like to automate this, to know if them, at least, read the email. I know this is very difficult due to how email works, but some companies already try to do this in a satisfactory way, like:</p>
  
  <ul>
  <li><a href="http://www.mailgun.com" rel="nofollow">mailgun.com</a></li>
  <li><a href="http://www.mailchimp.com" rel="nofollow">mailchimp.com</a> </li>
  <li><a href="http://www.sendwithus.com" rel="nofollow">sendwithus.com</a> (<a href="http://www.ycombinator.com" rel="nofollow">YCombinator</a> funded).</li>
  </ul>
  
  <p>In HTML mail messages we can create a resource that points to the server, like a image. But mail clients usually ask permission to the user to load the images. So, problem 
  here.</p>
  
  <p>But for text mail messages? Is there any way to know the email was read? How companies these companies do?</p>
  
  <p>PS: I don't know what tags is the best to classify my answer, I shall appreciate any edit.</p>