> Intermediate email forwarding is so rare as to be non-existent.
Where did you get this belief? Almost no e-mail user who uses a client to send SMTP right from their PC or mobile device can send directly to their destination, because they don't have a static IP address with e-mail reputation.
Directly to the destination meaning: looking up the MX record for the domain of target e-mail address, and contacting that host.
People who self-host such e-mail receiving hosts will themselves drop the connection from such a user, if they implement RBL-based anti-spam measures.
Where did you get this belief? Almost no e-mail user who uses a client to send SMTP right from their PC or mobile device can send directly to their destination, because they don't have a static IP address with e-mail reputation.
Directly to the destination meaning: looking up the MX record for the domain of target e-mail address, and contacting that host.
People who self-host such e-mail receiving hosts will themselves drop the connection from such a user, if they implement RBL-based anti-spam measures.