It appears that around the 3rd December Google implemented a change to how linked images are handled in Gmail webmail.
What can this mean?
Only the first user unique open may be reported. Reporting of your gross or total open rate for Gmail may drop.
Geo-location tracking and targeting impacted as location reported is that of a Google server such as Mountain View Device tracking shows the wrong device, in particular a drop in Gmail Webmail opens.
Real time content targeting or dynamic image testing broken as images are not fetched on every load
The change only affects new email campaigns. Any emails that arrived at the inbox before Google made this change won’t be affected.
The exact details of which services and solutions are affected and in what way is still being established.
However, if you are seeing your metrics show strange or usual trends then you should check with your vendors and IT whether the issue is related to this Google change.
What have Google done technically?
Links to images in emails are being replaced with URLs to Google’s own content serving network.
This means the first time a recipient loads a particular linked image in your email Google will fetch the image from your servers and then for all further loads of that image,by any recipient, the image is provided by Google’s own servers.
Open tracking images have unique files names or query string parameters which means these can’t be cached by Google across all users but have to be loaded on a per user basis. So at least the first load and hence first open tracking event should still be reliably recorded.
Services that use the IP address (Geo-location) or user agent string (Device) in the http image request to determine the image to be delivered can also be affected as to your servers the request will appear to come from a Google IP address and requesting device is a server rather than the actual users device.
Further technical details have been posted by Laura Atkins (Word to the Wise), Jeff Batte (DEG) and Andrew Bonar (Email Expert) who reports that some but not all email solutions are impacted.
These ESP solutions are reporting their open tracking is not affected:
Vendors with some impact are:
MailChimp have confirmed unique opens are OK but total or gross opens are impacted as predicted in this post
Litmus have confirmed unique opens are correct but you must setup a unique users ID in the Litmus tracking codes. Total opens are impacted.
Lyris also tracking unique opens but not total opens.
MoveableInk have confirmed that device targeting and geo-targeting based on detected location no longer work for Gmail webmail users
Geo-targeting based on IP address and device tracking solutions are impacted.
Andrew Bonar has posted more information about the fix.
Google finally posted on the 12th December why they have made the change to image caching. Its mostly good news, so they can safely move to images always on by default in Gmail webmail. Read the Gmail blog. A new Gmail help article also confirms that the Google change stops any device and location tracking using images.