I don't think it does. Has anyone made measurements? Text files and graphics don't have to be checked, only executable code.
That I agree with. The Internet was designed for use within secure research labs. It needs to be completely re-engineered for use by the public. We can't make authentication perfect, but we can make fakery a *lot* harder than it presently is. We can also make our software make more intelligent use of the routing information that is already included in everything we receive; phishing, for instance, is close to 100% detectable if anyone cares to do it.