I don't think I've ever had a working definition of a business rule beyond what feels right intuitively. I'm going to carry this forth with me.
Perhaps you've been reading this with mounting frustration: How about validating the address according to the SMTP spec?
Indeed, that sounds like something one should do, but turns out to be rarely necessary. As already outlined, users can easily supply a bogus address like [email protected]. It's valid according to the spec, and so what? How does that information help you?
I feel like this is the difference between an academic and a professional. One is trying to do it provably right and the other is trying to satisfy a need with limited resources.