Late last year Gmail started caching images on their servers, breaking open tracking in some circumstances. This image caching was good for senders, in that images were back on by default. But it was also bad for senders because it broke dynamic content and didn’t allow for tracking of multiple opens by the same recipient.
According to a new blog post by Moveable Ink this issue has now been resolved and Google is respecting cache headers so senders who are using dynamic content or want to track multiple opens can do so.
Last week, the Movable Ink team noticed that Gmail had begun deploying updates to address the issues caused by its 24-hour caching. The cache still exists, but it is now overridable if you pass a no-cache header (example below).
HTTP 200 OK Content-Type: image/png Cache-Control: no-cache max-age=0
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[…] in the inbox, Gmail is leaps and bounds ahead of the pack. They made some improvements to their image caching process and are now respecting cache headers, so marketers can update images and track multiple […]
I believe that the Cache-Control header in your example has a missing comma.