title: What technique is used in Bananatag Chrome extension to track email open
body: <p>I found this Chrome extension and very excited with this function.
  I want to know how this extension can do this.<br>
  When I open mail sent by Bananatag extension, I can see in the last mail body image tag: </p>
  
  <pre><code>&lt;img width="1" height="1" src="https://ci3.googleusercontent.com/proxy/5BkS5ptzSiTydfw6u4E3DlXN_g1UedSoHoCr_57prqFqPgk-8WMa8ZhpYhK2G3tLgPCa=s0-d-e1-ft#http://s.bl-1.com/j3zT7Vt.gif" class="CToWUd"&gt;
  </code></pre>
  
  <p>I copy url in src property to the address bar of browser and enter.<br>
  In the Bananatag member page, it generate records here!.<br>
  How can this extension do that, this url is <pre>ci3.googleusercontent.com</pre>It's Google server, how it can get request from Google server?</p>