Accepted means the server you send through took the message for delivery. It does not show that the message reached the inbox, and it is not a read receipt. A bounce shows that delivery failed. An image load or click shows that something requested the tracking elements, which may be a person or a machine.
The strongest sign of receipt is something the recipient does on purpose, such as replying. If you need formal proof for a contract or a court, use a service built for that, such as registered email or a qualified electronic delivery service, and check what your local rules accept.