We insert an image pixel tracker into the body of the email message. As soon as the email is opened and the ‘image’ is ‘viewed’ (most clients view images by default today, though some still request the user’s approval), we receive a notification via our API and increase the open count by 1. The user does not see the pixel and is unaware of its existence unless he or she is prompted to ‘display all images’ (though due to the recent trend of an increasing proportion of marketing emails heavy in HTML, most users view images by default). 

Note: If the pixel is triggered to display in any way by an email provider, it will track an open. (ie the email is forwarded to another recipient or viewed in your sent email). All emails that are viewed within Pipeline will not count an open.