> When you send a letter it does not belong to the post office while in transit. You only have an expectation of privacy of the contents of the letter. You hav

> When you send a letter it does not belong to the post office while in transit. You only have an expectation of privacy of the contents of the letter. You have no such expectation of anything on the envelope itself (e.g. sender/receiver address). The government contends that it only collects analogous metadata in transit, not the contents of the communication itself (although this is debatable).