The common analogy given from law academics is that email is more akin to the telegram than postal mail. With postal mail, no one along the entire path actually

The common analogy given from law academics is that email is more akin to the telegram than postal mail. With postal mail, no one along the entire path actually opens the envelope to see what is inside. This is in contrast to the telegram where the intermediary is exposed to the contents directly. This difference is what leads to the difference in expectation of privacy. In this analogy email is actually more like a telegram. Where I think that the analogy diverges with reality is that the intermediary is now a machine, not a person. Therefore, we expect the machine to not "understand" the data that passes through it and simply deliver it.