Step 1:  The receiver simply presses reply to the e-mail.  NOTE: The sender has the option to send the e-mail with or without the Registered E-mail banner.

Step 2:  Once the "Reply" button is pressed the recipient can (1) insert any text requested by the sender and (2) press the "Send" button.

Step 3:  The original sender gets a Registered E-mail with (1) the reply text and (2) the original e-mail below.  Both parties are sent a Registered Receipt e-mail of the entire back-and-forth e-mail communication.