title: Images in Gmail not displaying due to Google Image Cache / Proxy
body: <p>I have a script to send emails to subscribed users and is currently facing issues with images not displaying properly for users using Gmail.</p>
  
  <p>The images used in the email are publicly available, e.g.:</p>
  
  <pre><code>https://info.xxxx.vn/banner/2018/xxxx-600x300.jpg
  https://info.xxxx.vn/banner/2018/xxxx2018v2-600x300.jpg
  </code></pre>
  
  <p>When the emails are sent to a Gmail user, the image link is changed by Gmail to:</p>
  
  <pre><code>https://ci3.googleusercontent.com/proxy/E6ElwPPH-lH4YdxOtzc8rl9GUpcPA_Kf1E0UFDxNT877wV70lA-KgwQDYQhTXU8ed7U9HJF21PMmGiv0M93jnAKvHszQcFdVAf7ApQta2QgQ=s0-d-e1-ft#https://info.xxxx.vn/banner/2018/xxxx-600x300.jpg
  https://ci5.googleusercontent.com/proxy/xK0x7LIhvH9Inax29k0bZKsnC5FerVEhNXjyleL0lU0tbXgb2RBLOYcy_j834lJlOISKgwT6yzJmPWCuHGb9wBkycdEWvwGgWPWJ87NGAJbrosh6gibo9A=s0-d-e1-ft#https://info.xxxx.vn/banner/2018/xxxx2018v2-600x300.jpg
  </code></pre>
  
  <p>Each of the image links above returns response 404.</p>
  
  <p>Things I've tried: <br/></p>
  
  <ol>
  <li> Use images uploaded to AWS S3 for displaying in the email:<br/>
     - It displays perfectly in the email </li>
  <li> Disabling any firewall that might block crawlers from accessing the domain: <br/>
     - Image is still not displayed. <br/>
     - Network team did not notice any external request made to the URL
  </li>
  </ol>
  
  <p>Is there a way for me to know the exact error which is returned by Google when trying to cache the image? Right now it is always showing the 404 error and there's no way for me to troubleshoot the exact issue.</p>
  
  <p>I would really appreciate any other input as well which would help with troubleshooting the issue I am facing right now.</p>
  
  <p>Thank you very much in advance.</p>