We track opens on all html-based campaigns (emails created using the drag-and-drop designer or your own custom code) by embedding an invisible, 1-pixel image in your campaign. When a contact opens your email and chooses to display the images in your email, that graphic is downloaded from our server and it's recorded as an open.
If a contact chooses not to display images, their open may still be tracked if they click on a link contained in your campaign or automation email. Note that link tracking will need to be enabled for this option.
Note: It is not possible to track opens for plain text emails.