Gmail deploys image proxy servers

This afternoon Justin Foster of LiveClicker posted to the OnlyInfluencers list asking about Gmail rewriting links.

Sometime very recently (last 24-48 hours), we are seeing that Google made a change to Gmail such that all image URLs in the email content are replaced by a call to Google’s content caching service googleusercontent.com.
For example, an image with the src: “http://mysite.com/i.jpg” will be replaced by Gmail with a URL something like this: https://ci3.googleusercontent.com/proxy/…#http://mysite.com/i.jpg

After some investigation, testing and talking with people at various ESPs, I can confirm that Google is rewriting image links. This rewriting appears to be happening during the delivery process. Older messages that are currently in mailboxes aren’t showing this tracking.
Many marketers are concerned about this. The first concern is always about open tracking and how this will affect engagement metrics.
Normal open tracking happens when a user opens an email and loads images into their mail client. Each email address is given a unique image name so that the sender knows who loaded the image. Every time a user opens the email, the image is reloaded from the image server.
In the new Google setup, the first time an image is opened, Google downloads the image from the image server and caches it on a Google managed proxy. This means that the first image load can be tracked by the sender, but any subsequent image loads will not be tracked.
For senders, this means that only the first open of any individual image will be recorded. When someone opens a mail, Google will check to see if that image is in their cache, if it isn’t, then they follow the link, load the image and put it in the cache. Any time someone tries to load that same image, whether the same or a different recipient, Google will serve the image from the cached page.
For global images, this means only one user has to open the mail and the images are pulled from the server. In the case of tracking images, every image file name is unique. Every new open will cause Google to grab the uniquely named image. The result is that senders can track the first open, but no subsequent opens.
We identified the following string from an open at Google

66.249.84.36 - - [05/Dec/2013:13:00:55 -0800] "GET /zimbabwe.png
HTTP/1.1" 200 2867 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1;
de; rv:1.9.0.7) Gecko/2009021910 Firefox/3.0.7 (via ggpht.com)"

which is a Google proxy string.
Images aren’t just used for open tracking, however. There are a number of services which provide geo-specific images depending on where the images are opened from. This new proxy is going to break that. I’m also hearing of at least one email services provider that is seeing no opens from Google today, possibly because of how their images are interacting with the proxy server.
In any case, this is an issue we’ll be keeping a close eye on.

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