The collection, mainly through electronic means, of vast amounts of personal data is said to invade privacy. But machine collection and processing of data cannot, as such, invade privacy. Because of their volume, the data are first sifted by computers, which search for names, addresses, phone numbers, etc., that may have intelligence value. This initial sifting, far from invading privacy (a computer is not a sentient being), keeps most private data from being read by any intelligence officer.
You can't open up my mail (or e-mail) and look for names, addresses, and phone numbers without violating my privacy. The courts have held that e-mail headers are like pen information on phone calls, but the contents of the message are private. Get that? It doesn't matter what or who in the government opens my private communication, they've opened it and it's a clear violation of the Fourth Amendment.
You've got to have a warrant, mister, with a good reason, signed by a judge (hopefully not Posner), describing exactly what you are looking for and why you think you have sufficient cause. You can't open up everyone's communications fishing for crimes.
P.S. Richard Posner later writes during a chat:
I don't think most people would mind the government's scrutinizing their conversations for information of potential intelligence value if they trusted the government not to misuse the information.
Uhh... yeah, we mind. This fellow probably shouldn't be a judge based on his poor reading of the Constitution. It's not what "most people" would mind, it's whether these things violate the Constitution. And they do.