I know there is some ways to know if an email has been opened such as return receipt which is "blocked" by default in most email clients nowadays. There is also url or image tracking (with gmail downloading images automatically).
I would like to know if you can think of any other, legit or not, ways to get acknowledgment of the opening of an email ?
Edit for clarity : my concern is more from the receiver's point of view. When you open an email, is there any ways for the sender to know it was opened aside of url/image tracking ? I'm on gmail with defaults settings, but interested in other clients as well