The opening rate of a campaign is calculated based on an invisible pixel/picture within the email. It is invisible because it merges with the background color of the message.
A pixel works in the same way as a cookie on a webpage. Whenever a recipient opens an HTML message, as soon as pictures are downloaded, so is that pixel. It sends a request to the emailing server and this piece of information is recorded as an opening of an email.
For this reason, we recommend that you include a mixture of text and images in your message so that when the recipient opens your email, the pixel downloads along with the images in your message. This means that the opening is detected.