How often are you validating email addresses for localhost!? Those are pretty much never a valid email for >>>99% of applications
Right, so if you're not going to accept the full RFC then where do you draw the line? Your arbitrary line will assuredly be different from someone else's.
That's the whole point of the original post: don't bother thinking about it, let the mta do that for you. The "@" is a courtesy to make sure people didn't mistakenly put their name in. Don't validate, just do a sanity check.
And people are just as likely to type an valid but incorrect email address as a invalid email address.
That's why you also implement an email verification method if you actually care that it's a correct email as well as valid.