How does email client caching affect metric accuracy?

Email client caching changes how and when tracking pixels load. When a client such as Gmail.com or iCloud.com downloads and stores images on its own servers, the pixel may load only once rather than each time the subscriber opens the email. This reduces the accuracy of open counts.

Some clients also prefetch images before the user even sees the message. This can create artificial opens. The ESP sees the lantern flash even when no one has looked through the porthole.