How are opens tracked in email reporting?

Opens are tracked by a tag included in the HTML version of your email campaign that records an open when the tag is triggered. The tag renders a transparent 1x1 pixel image and this act is recorded as an Open. An Open is also registered if a recipient previews the email message in a preview pane.
 
Note: As tracking opens relies on an image render it is not possible to track opens for text-only or text versions of the email messages. Additional image blocking in the email client will prevent an open from being recorded.