Return receipt is a feature by which you can get an acknowledgement (email) when the recipient reads the email that you have sent. To get a return receipt, click on 'Return Receipt'.
When the recipient opens your email, he/she gets a pop up saying:
“The message sender has requested a response to indicate that you have read this message. Would you like to send a receipt?” Yes | No.
You get a message (example): “This is a receipt for the mail you sent to