Replying to all inbound messages with a bounce is an old idea. The theory was that legitimate senders would validate their address and get added to a white list. The scheme didn't work for the same reason captchas don't work.

Maybe an old idea and you can feel about it however you want, but it's still in universal use right? So it's not quiet or am I misunderstanding what you're saying?

Lots of big email services do quietly reject mail. They successfully accept it but then throw it away (sometimes not even in the spam folder for particularly high span scores).

That's their issue. Don't be a customer at such an email provider :shrug:

I had a legal dispute once where a shop didn't provide warranty on a product. They took the product in but then it was indefinitely in repair. Long story short, it ended up being helpful that I have email server logs that show every transaction, so that I could say "on date+time the email was delivered to IP address X, which was registered as the mail exchanger for your domain, and your server took the email with status code XYZ Queued <ID> For Delivery." I got the money back after that

Afaik that's not what Microsoft/Hotmail does though: since it rejects, you get a delivery failure notification

That's not a rejection then, it's a discard. Terminology matters.

Sure "silent rejection" == "discard"