When you send an email to your users, Keepnet Labs places a user-invisible image with a height and width of 1px at the bottom of the email to see if the email is read by the target user (sometimes called a tracking pixel). This is a very common practice for modern digital marketing campaigns. However, many email services automatically block images, therefore resulting in the "red X". If the user downloads the image, a request is sent to the address where the picture was is originated from. Thus, our portal can report whether the email has been read or not, provided the user 'downloads' or 'opens' the picture.