Suppose you are sending mass emails (legitimately, no doubt) and want to know which % of recipients actually viewed the email. The standard trick here is to embed a 1x1 image into your email’s HTML source, with the <img src= pointing to a location on your Web server with part of the URL unique to the user (e.g., <img src="http://example.com/track/12345" /> where your mailing system knows that 12345 is associated with john@example.org). When the user opens your email, most email...