It's within RFC, but they all lead to one email inbox so you end up being able to manage multiple third-party accounts from a single email account. It's recommended to not reject these, but strip them: https://gist.github.com/judge2020/af8fb9cd2ac165462d44de4e58...
Yes, I know that. And as I mentioned, companies are still rejecting these addresses anyway because they know people are using them to identify and filter spam.
This is why I use a catch-all on my own domain with a blacklist for companies found sharing or leaking the email address I gave to them. Fastmail makes this really easy to set up and their web interface also lets you set the From address to anything on the domain.
How rich.