I take it as being RETURN TO SENDER - the sender having been the lawyer whose name is shown.
I take it to be the postcard that is returned to a sender whenever something is mailed via certified mail, return receipt requested. The signed receipt means that the document(s) to which it was attached has been received and signed for by the recipient. In this case, the divorce papers were received and signed for by BHO Sr. The post office then returned to the lawyer the receipt (both sides shown) with BHO Sr.’s signature, proving that he was served with the papers, because he signed the receipt (unless that’s not his signature).