Creating one. But when you consider just how many customers are using gmail and outlook addresses, and not to mention, GSuite/fastmail/etc. addresses under custom domains, it makes more sense why rejecting @[email protected] is worth more than allowing some crazy e-mail feature that is effectively not used.

The routing features are obsolete; they go back to the days when lots of email users weren't on the Internet directly and had to use relays. They are still in the spec, yes.

I assume it comes from similar lineage as UUCP paths. Either way, email standards are a bit ridiculous. It needs the kind of rehaul that occurred with HTML5 of looking at what email implementations actually do and pushing them in one direction. I suspect that is not happening ever, so failing that there will probably always be things in the spec that just simply don’t work across everything anymore.