The issue here isn't about knowing whether the the recipient read the email, which is not an interesting thing to spy on.
It's about the sender seeing whoever the recipient forwards the email to, and about the sender seeing the recipient's network information (although that's a hard sell because the recipient already advertised their network presence by using email), and the use of non-HTTPS servers that could be compromised to intercept traffic.
It's about the sender seeing whoever the recipient forwards the email to, and about the sender seeing the recipient's network information (although that's a hard sell because the recipient already advertised their network presence by using email), and the use of non-HTTPS servers that could be compromised to intercept traffic.