A return receipt is a message that tells you that someone has opened an email you sent them.
When the person opens the email, you get a message in your Inbox with the subject, recipient, and date & time he or she opened the email.
Here is an example return receipt:
If you want to get return receipts for an email, you can click Delivery Options
while composing the email.
If you want to get return receipts for all emails, see Always get a return receipt