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Gmail’s recent changes to image displays and the impact on email tracking

Over the course of December 2013, popular webmail client Gmail have released a brand new way of handling images embedded in emails. The so called “image caching” method was initially soft launched for Gmail users as early as October 2013 (those with an address) and extended in December to Google App Engine customers with own domains using Gmail as their mail client.

The good news first: with the arrival of image caching, Gmail is now by default showing images that are embedded in html email, making the oh-so-annoying “always display images by this sender” prompts obsolete by turning it into an opt-out scenario[1]. Whether or not the developers were trying to make up for the alarming open rate drops after the introduction of tabbed inbox categories in May 2013, this is a welcome improvement for all email designers and e-marketers who are relying on a highly visual appeal of their B2C communications. Depending on cleverly styled alt text and complicated mozification of images – at least for messages opened in the native Gmail client (web interface or its smartphone and table app) – is a thing of the past. It opens opportunities to create even more engaging campaigns with the freedom to use more screen real estate for brand and product design. And with Gmail already the fastest growing global ESP[2] which has proven to set layout, inbox management and email triaging trends previously, other web based ESPs are likely to follow.

But that’s almost where the benefits stop and the head scratching starts.

What is image caching

Upon its broadcast an html email hosts all images that are embedded in the html on a third party server. This can be the web server of the sender domain or the Marketing ESP’s cloud server (i.e. Campaign Monitor, Mail Chimp, Exact Target etc.). Until recently, that’s exactly where the images stayed for everybody – incl. Gmail users.

What Gmail has started doing now is to store copies of the images on their own proxy server when a message is first opened, then serving the images from this cache for every re-open of this email on any of Gmail’s native applications. The advantage is a faster loading message with images “always on”.

 The big But

The downside is two fold. Because images are stored and loaded from Google’s own proxy server immediately after an email is first opened, any server side or dynamic changes to the hosted images after the broadcast won’t be visible to Gmail customers. An image swap after the event – i.e. to react to topical or last minute changes or even dynamically served alternative images at different times of the day – will not reach recipients using any of Gmail’s applications.

Users who check their Gmail messages outside of the native tools (original browser based webmail, Android and iOS apps) are not affected by this behaviour.

The second and more significant drawback of image caching is the reduction in tracking capabilities.

Email analytic services almost exclusively work by serving a tagged 1-pixel image which carries a unique identifier in its file name or query string. This add-on allows email marketers to get insight into the recipient’s location and device(s) used when an email is opened, as well as the number of total opens by each user. This helps a great deal with tailoring campaigns to target markets, optimising user experiences and gathering reliable information about an audience’s interaction with a brand.

As soon as an email is opened in Gmail and its images are stored on the Google proxy server, the references from the original html are being manipulated to point to the cache with all unique tracking specifics pointing to itself. Any repeat interaction with this email will not be captured and subsequent opens – or re-opens – cannot be tracked.

Gmail's image caching

The official Gmail usership is estimated at approx. 6% worldwide. Rather neglectable one might assume. But now look at almost 10m unique monthly visitors on Gmail’s browser version in the UK alone (leaving out mobile and Google Apps users), it’s an audience that’s difficult to ignore. Gmail is also the fastest growing webmail client. (source: ComScore Data Mine)

The solution

It’s not all doom and gloom for the future of email marketing. Many clever articles have already helped to raise this issue and brought it to the industry’s attention. Thanks to organisations such as eConsultancy, Litmus, Silverpop, MailChimp, A bright developer, René Kulka, has now allocated the area for a fix and some very few ESPs have since applied it to their tracking method. Read René’s brilliant findings here.

The HTTP header information of the tracking pixel that is produced needs to be looked at. More precisely the value of the Content-Length, which must be “0” for caching by Gmail to be prevented

Here’s an example:

If the current tracking pixel header looks like this:

HTTP/1.1 200 OK
Server: XXX
Cache-Control: private
Content-Length: 35


It needs to be changed to

HTTP/1.1 200 OK
Server: XXX
Cache-Control: private
Content-Length: 0


This fix seems to avoid Gmail image caching altogether without losing the attractive feature of the default image display. However this HTTP amend needs to be made at the ESP’s tracking backend and cannot be manipulated by the broadcasting party.

It is highly recommended to all reputable ESP and eMarketing organisations implement this fix a.s.a.p.

Without this change, expect a positive effect on open rates and a negative effect on total opens, real-time content serving and device usage reporting.



[1] You can still chose to not display images by default, however you have to manually apply this setting.

[2] Email Service Provider

Categorized: talking web

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