Please note that the concept of "return receipts" (RR) are very outdated.
They were originally conceived when the internet was young and people actually needed to see if email made it from point A to point Z. As the infrastructure got better, the increased traffic from a RR got annoying (you figure every email became 2 emails), and the reduced actual need meant that more and more 'modern' emailers didn't include the facility for RR. Also note that in order for an RR to work, the recipient's SMTP server has to be configured to acknowledge RRs and preserve them in the mail's header, AND the recipient's email client ALSO has to allow the use of RRs. So..
If you try it and it doesn't work.. there's a ******** of variables at play here.