Username or EmailPassword
Indeed, it can't be all that hard to do a search of a bounced-back email like that for keywords which trigger a response template like the one shown. Especially Apple, as the kings of polish and facade, I would have thought would sort that out.
I know different mail servers will respond differently, but they'll generally follow the same patterns. It's able to do it with networking protocols like Samba (which also differs in its response from server to server) so why not with mail?