Embed some piece of HTML that calls back to the URL of an external server. Each recipient gets a slightly different URL. So when the email client parses <img sr

Embed some piece of HTML that calls back to the URL of an external server. Each recipient gets a slightly different URL. So when the email client parses <img src="server<dot>com/emailtracker.php?id=abcd"/> it connects to server<dot>com/emailtracker.php and passes your ID (abcd) to the emailtracker.php page. Then some code in emailtracker.php finds your ID in a table of ID's issued by sent emails and records your IP and browser fingerprint alongisde the ID they issued you when they sent the email.