Probably something like <img src="http://google.com/pixel.gif?mailid=23949129"> http://google.com/pixel.gif?mailid=23949129"> in a mail. When you download the i

Probably something like <img src="http://google.com/pixel.gif?mailid=23949129"> http://google.com/pixel.gif?mailid=23949129"> in a mail. When you download the image, Google knows you have read the mail.