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

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.