The ReturnReceipt property enables or disables delivery
status notification (DSN) by the mail server. If the property is set
to True, a mail message will be automatically returned to the sender
indicating if the message was delivered successfully, unsuccessfully
or delayed by the mail server. If the property is set to False, no
delivery status information is sent back to the sender.
Note that delivery status notification is not available on all
servers. It is also important to note that a message indicating that
delivery was successful does not mean that the message was actually
read by the recipient, only that it was delivered to their
mailbox.