More info about Gmail image caching


A lot of people are discussing the new Gmail image caching around the web.
This doesn’t yet appear to be rolled out across all of Google’s network, so some people in different parts of the world are reporting different behaviors. This is leading to a little bit of confusion, as folks are reporting things like seeing multiple opens for a single image. These reports are clearly accurate, but may only be an artifact of a slow rollout across the network.
There are a couple bullet points I think are important.

  • This does not affect tracking of the first email open as the cache fetches the image from the sender.
  • This does affect tracking if someone opens an email more than once as the email client is pulling images from the Google cache.
  • This does affect geolocation detection as Google is not providing any information about where the open happened.
  • This does affect user agent detection as Google is not providing any information about the original user agent string.
  • This does affect device detection as most of that is done by detecting the user agent.

Derek Harding tested whether or not Google was respecting the expires or no-cache headers. He served images with headers including

Cache-Control: no-cache, no-store, must-revalidate
Pragma: no-cache
Expires: 0

but Google still cached the image.
Right now there doesn’t seem to be any way to bypass the image caching step. I am also not sure how this is affecting people reading mail on standard mail clients ( or apps on mobile devices.
Other articles discussing the Gmail image caching
Zettasphere: Google Gmail change Breaks Email Open Tracking
ExactTarget: Gmail Now Caching Images
MailChimp: How Gmail’s Image Caching Affects Open Tracking
EmailExpert: Gmail Breaks Email Marketing Again
Frenzy Commerce: Gmail image changes: everything email marketers need to know

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  • Thanks for the synopsis. I would say, point two is inaccurate “This does affect tracking if someone opens an email more than once as the email client is pulling images from the Google cache”.
    It may affect tracking it may not. In my tests it did not on at least one EDM platform. I am just creating a video now to illustrate this point, as I see almost everyone is presenting this same viewpoint as fact. I will provide a link to the video, which clearly shows the tracking of multiple opens from a single device in just a few seconds.
    I do not doubt many platforms are having issues, however the open tracking issue is NOT ubiquitous industry wide, despite what some may wish you to believe.

  • I’ve done some further investigation of the fix. Essentially the approach is to have the mime headers of the HTTP request lie about the content. e.g. have the headers say the content is HTML but have the content actually be an image.
    This kind of fixes the multiple opens issue. I say kind of because the google cache is still preventing the no-cache headers going through so a user’s browser will often cache the content locally when it otherwise would not. THe result is that multiple opens will still be under-reported and dynamic images not always updated, but less so than without this fix.
    It also does nothing to address the other issues around user agent, location, referer and cookies.

By laura

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