To determine whether a message was opened, an invisible tracking pixel is added to the HTML message (see How Mapp Engage Measures Email Marketing Success). The tracking pixel can be added either at the beginning or the end of the message.
The tracking pixel is only seen by the recipient if the email provider suppresses the display of images in the inbox. In that case, the tracing pixel is displayed as a missing image.
If the pixel is added to the end of the message, this is less noticeable for the recipient. Furthermore, this guarantees that an open is only counted when the message is completely loaded. The pixel is only uploaded when all other pictures are also uploaded.
If the pixel is added to the beginning of the message, even brief opens are counted. This can make a difference above all by slow internet connections.