Please don't implement this sort of system---it's a headache, and can hurt users more often than not (important email from many important sources is unlikely to

Please don't implement this sort of system---it's a headache, and can hurt users more often than not (important email from many important sources is unlikely to be authenticated, say a recruiter, or a contact for a new job, or a distant great-aunt who can't use computers), plus it's really annoying when, for example, mailing lists get this sort of annoying check back almost every time they email a new user.