A transparent tracking image is automatically inserted into the header of all emails. When a contact clicks on a message, either to open it fully or view it in the reading pane, this transparent image gets downloaded. This acts as a read report and, when the receiving server allows this report to be sent back to the sending server, the system is able to track that message as having been opened.
Note: If the recipient’s server blocks images, we can’t track that the email’s been opened. However, if the contact clicks on a link, then we record this as an “inferred open”.