It was "solved" in some general sense but was never implemented widely or consistently. It was traditionally a really clunky and fragile system (e-mail forms)

It was "solved" in some general sense but was never implemented widely or consistently. It was traditionally a really clunky and fragile system (e-mail forms) that ended up being really limited in its utility because it wasn't widely used. That said, the point isn't that this is a technically difficult problem (it isn't) but rather one of human scale: altering the behavior of tens of thousands of more-or-less autonomous networks spanning the globe is non-trivial.