Zeta Blog

Gmail Image Caching Update

Posted by Satu Rehumaki on April 11

In a series of blogs posted in December (“Gmail’s War on Christmas”) we alerted you to a change in Gmail that had a significant impact on email marketers. Google quietly began to proxy images through their own servers, impacting the way they were cached. The change was intended to enable Gmail to display images to users instantly without the need to click a “view images” link. But in doing so it inadvertently prevented marketers from tracking re-opens and stopped real-time content from being served upon re-opens.

In order to understand how a potentially beneficial feature like instant image display could go so wrong, you first have to understand how image caching works. Email image caching is controlled through the use of headers that accompany images. The image header provides the email client with information that dictates how long an image should be re-used before making a fresh request to the server. This is key for serving real-time content.

When Google began using its own proxy servers, Gmail continued to recognize the header caching instructions sent from the original server, but it passed the images to email recipients with instructions to re-use the same image for 24 hours. As a result, email clients would continue to deliver real-time content on the initial open, but not on subsequent re-opens until a day had passed. Because open tracking relies on images, initial opens were registered but re-opens were cached and couldn’t be tracked. Since the change’s initial introduction, Google has deployed updates to address the issues caused by Gmail’s 24-hour caching. The cache still exists, but it can be overridden if you pass a no-cache header.

As a result, re-open tracking now works as it did before the Gmail changes of December. Recent Gmail updates have also addressed a long-standing issue of temporarily caching entire emails when navigating between. Now whenever your recipients open an email in Gmail, you can be sure that they are seeing the most current, real-time content with every open and re-open.

Topics: Email Marketing