It's a small transparent pixel loaded from a website (assuming the email client loads HTML). I'd make a unique image called mywebsite.com/jaclaz_read.png and when that image was loaded I'd know you read my email.
It's a small transparent pixel loaded from a website (assuming the email client loads HTML). I'd make a unique image called mywebsite.com/jaclaz_read.png and when that image was loaded I'd know you read my email.