Why not have a simple validator that works with 99% of users emails but not make it mandatory that it passes validation? "We see that bob@localhost doesn't loo

Why not have a simple validator that works with 99% of users emails but not make it mandatory that it passes validation? "We see that bob@localhost doesn't look like a email address are you sure it's right?" That way you can help users that messed up their email but not prevent all the corner cases. The idea is that most email addresses fall in a very narrow subset of the RFC: user@domain.tld and most people would have entered their email wrong if it didn't match that pattern.