I never used 'return receipt'. Did that let you know whether the recipient opened the email you sent them?
That was my understanding. I’ve never used it on Gmail, but have occasionally encountered it in other mail implementations, particularly the Notes implementation at a company I once worked with.
The question I don’t know is whether it is automatic or voluntary. There is a topic “return receipt” on Wikipedia, which makes it sound voluntary. That is, a successful delivery can be reported, but not necessarily the opening of the message. I’m sure someone more knowledgeable on this can chime in.