title: Email integration
body: <p>I was wondering if someone could help me out.  In some web application, the app will send out emails, say when a new message has been posted.  Then instead of signing into the application to post a reply you can just simply reply to the email and it will automatically update the web app with your response.</p>
  
  <p>My question is, how is this done and what is it called?</p>
  
  <p>Thanks</p>