>...or else you send it directly from your machine your friend's SMTP server, using TLS, ...
That is normally what happens these days. Intermediate email forwarding is so rare as to be non-existent. Only a small percentage of email is sent unencrypted between servers[1].
Agreed that email should be encrypted and, perhaps more importantly, signed...
> 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.
That is normally what happens these days. Intermediate email forwarding is so rare as to be non-existent. Only a small percentage of email is sent unencrypted between servers[1].
Agreed that email should be encrypted and, perhaps more importantly, signed...
[1] https://articles.59.ca/doku.php?id=pgpfan:starttls