Recently I wrote an application to analyze if an email was opened. In a brief, the email contain an image and when the receiver open it, the image is loaded in my server and I my code do some analyses and register it.
But, recently Google created an image proxy server. It means that after the first request the image will be downloaded from Google and not from my server, so it broke my idea. (of course, just for gmail users)
However I did a very simple workaround, all images that I am sending have a unique name now. So Google won’t be able to cache it after the first request. Cool!