With its latest change in Gmail, Google is caching most of the images contained in email messages sent to Gmail users. It is a technical change that makes email operations more energy-efficient, as the email servers do not have to retrieve an image every time a message is read. The energy savings may add up to a few cents if the same message with the same images is sent to thousands of Gmail users, and these savings are shared by the sending email servers too.
The real reason for the change, though, may be security. Bulk emailers use dedicated web servers to collect data on the way people interact with email messages by tracking the images embedded in the messages. Spammers routinely use this information to determine which of the email addresses they send to are still active. With image caching, such information is much harder to collect and analyze. This makes it harder for spammers to target you. But more importantly, it makes it harder for anonymous email senders to collect data on the way the Gmail system works. Such data has been used in the past not just by spammers, but by the Chinese military, the NSA, and other political and industrial espionage organizations in their efforts to break into people’s Gmail accounts. Images in email still represent something of a security hole, but Google is making that opening smaller.