Most email providers and programs nowadays do not display images and other data linked to third party servers by default when you open emails on their websites or in email clients.
If you are using Thunderbird for example, you will receive a "show remote content" notification if an email attempts to load contents from an external server.
The core reason why it is handled this way is privacy, as remote connections, for instance to load an image from a third party server, may reveal information about you.
So called header information are included in each transfer, and each header may reveal information about your operating system or language. In addition, since a connection is made, your IP is usually also logged by the remote server.
That is not a problem if the email was sent by a friend or someone you trust, but spammers use the same method to retrieve information about you.
Block images on Gmail
Gmail up until now displayed an "Images are not displayed" notification for emails that contained images. The system provided you with options to display the images below, and to whitelist the sender of the email so that images would be displayed automatically in future emails.
Google announced a change to the way images are handled on Gmail. Starting with today, images are automatically displayed in emails thanks to a new proxy technology that it has implemented on the site.
Images that are included in emails are transferred through a proxy automatically by Gmail, and virus checked among other things, so that they are not loaded directly from a remote server when you open the email, but from a Google server instead.
This effectively removes the need to connect to a remote server when images are included in emails that you read on Gmail.
While it is certainly improving the privacy, some users may be inclined to turn the new feature off for two main reasons:
- The images run through Google servers before they are displayed to Gmail users in emails. It is not clear if they are cached by the process as well.
- Since images get loaded automatically, it may take longer to display emails on the Gmail website.
Gmail users who did enable the "Ask before displaying external content" option explicitly on Gmail, will have the "ask before displaying external images" option enabled by default.
Everyone else needs to enable it under Settings > General before it becomes the active preference.
Doing so will block images from being displayed inline automatically when you view emails that contain images.
For now, it appears that Google is loading the image when you access the email, which in turn provides the sender with that information. It does however strip the user-agent headers, the user's IP address and cache-control headers from the response.
A proxy is definitely a solution against spammers and marketers that try to collect information about users by loading contents from third party servers.
It has the side-effect that many "check if the email was read" services that use images to do so won't work anymore if the recipient is a Gmail user and using the Gmail interface.
It is not clear if the new image proxying technology is also working when Gmail users are using third party clients to retrieve emails, or if those services and clients are exempt from this.
The feature is currently being rolled out for all desktop users of Gmail. Mobile users will receive the update in the beginning of 2014.