[anigbrowl]

Just because you haven't heard of it doesn't mean it doesn't happen. Here's a case from 1970: http://scholar.google.com/scholar_case?case=1776184466461190...

You'll notice that early on, it mentions Ex Parte Jackson, an 1878 case which established that the contents of mail are protected by the Fourth Amendment, but that the outward form is not. Mail 'metadata' has always been fair game, just as it would be fair for a police officer to observe your comings and goings on the street without any need for a warrant.

Then there's the Postal Inspection Service, which is the law enforcement agency that specializes in mail fraud (perhaps you've heard of that?) and which predates the founding of the USA.

I don't mean to be rude, but the fact that you've 'never, ever heard of' something doesn't mean anything special. You don't strike me as terribly well-informed.