> Email is extremely non-brittle, and everyone following the RFC is one main way it is able to be non-brittle. What is naive here, is that you think things are the way there are, for no reason.
> Mail is decades old, built upon millions of hours of work crafting software, RFC standards, and works the way it does, including bounces, to ensure stability.
Same people argue that email is a hot mess exactly because it's decades old and comprised of a patchwork of standards. It's not exactly the poster child of how to do this sort of thing well.
> For a mail system to respond with 5xx, when it is internally broken, is 100% a configuration issue. Every MTA on the planet, is designed (eg, postfix, sendmail, etc) to respond with a 4xx TEMP fail message if something is borked. A milter gone bad. A library missing. A full disk. A config issue. An issue forking. Memory limits. All of it.
Configuration issues can be, and often are, temporary. Also, bugs are a thing.
> THAT is where the "good engineering" failed.
I agree with this.
> What you want to do is make 5xx like 4xx, because you feel there should never be any way for a SMTP server to say "No, really, this email address doesn't exist.. don't bother trying again".
Of course there's a semantic difference between "come back later" and "go away forever", I'm just arguing that a client that doesn't blindly trust what every connected system tells it is going to more often successfully achieve its goals than one which does.