I think the author is stretching the words "valid" and "invalid" past their limits here for the sake of hooking you into the article. Yeah, in some countries it

I think the author is stretching the words "valid" and "invalid" past their limits here for the sake of hooking you into the article. Yeah, in some countries it's a valid social practice to spit on the floor in public, but in most, it's not. let's say I'm a dev at google, and I'm writing some aspect of gmail. Is !"£$%@gmail.com a valid email? No. So the word valid is clearly not being used correctly here. At the core of smtp these emails are allowed, but in practice they almost never are, and so the opposite is true. All the cases he described are, in practice, invalid.