Do you know what an email tracker is?
It's way more than a "read receipt" attached to your email. There are programs that watch & monitor EVERYTHING you do with your emails.
When you opened it...
How long you read it...
Did you re-read it...
It's all right there!
Check out this crazy BuzzFeed piece on how a Jay-Z super fan is tracking his emails to the superstar. He has NEVER got a reply... but he KNOWS someone is them. Possibly even Jay Z.