When a campaign email is sent, we insert a tiny, invisible graphic (called a web beacon) at the bottom of your email. When the customer opens the email, that graphic is downloaded from our system. That's how we are able to know when the email was opened.
Open tracking is not guaranteed to work. Some email clients will block graphics by default, especially if your email address is not in their address book. When that happens, the graphic is never downloaded so we do not know that the email has been opened.