Our email tracking system embeds a small invisible graphic in every message.  When an email is opened, this graphic is downloaded. Our tracking process can determine the individual download of each image by each email account, thus determining the open status of that message.  Some email clients do not automatically download images. If an image has not been downloaded, we can still tell if an email has been delivered or not, based on the response from the user’s email server, however we may not be able to detect if that message was opened.  We can also tell if there has been a problem with mail delivery, again based on information received from the user’s email server.  We can accurately track your email traffic, and if an error occurs it will tend to underestimate the number of opened emails. 

The image added to the image is a single pixel in size and uses a negligible amount of data. It does not add meaningfully to your bandwidth load, even if your organization is sending a high volume of email.  If you have added graphics to your email, this will not count toward our count, as they are not being downloaded from the same location as our invisible open tracking image.