An email open is tracked when a link is clicked or a 1×1 pixel graphic (called a beacon) which is inserted into the email is downloaded. This only happens when a recipient opts to 'download images'. If an email is opened, but the recipient does not download images, it will not register as an open. It's a universally accepted metric within the email marketing world that open rates can be off by up to 35%.
Our research shows that on average 14% of emails sent to the education sector are opened, but are untraceable as the recipient has chosen not to download the images.
To give you as accurate a picture as possible, Campus's school emailing statistics include the 14% researched figure of untraceable opens.