A missing read receipt shows nothing, because the recipient or their program can decline to send one. For important declarations with deadlines, such as a termination, ask a lawyer which form of delivery and which proof suit your case.
HawkOpen does not use read receipts. It records when your sending server accepted the message, whether it bounced, and later image loads, clicks and replies. Acceptance by the sending server is not delivery to the recipient, and an image load is an estimate, not proof of reading.