Stack Overflow emails use Javascript for email tracking of opens, clicks, and unsubscribes or may contain a small, invisible, one pixel image which can be used to let us know when the email is opened or forwarded. If the email recipient has images enabled on their device, when the email is opened, a message (an automatic request) is sent to Stack Overflow’s server requesting the image. That request is logged by Stack Overflow and tells us that the email has been opened. A similar process may be used in relation to links or attachments in the email, with a request being sent to Stack Overflow for the content that is accessible via the links or attachments, thus telling Stack Overflow that the link or attachment has been accessed.