At least under the 4th amendment, expectation of privacy must be objectively reasonable, not subjective. I.e. If everyone is misinformed about how email is sent around in plain text, and expect privacy out of ignorance, then that belief can be objectively unreasonable even if it's widely held.
Where did you get that information? From Wikipedia? Could you put the paragraph + link here so I can find it? I ask only because I looked it up myself and couldn't find that reasonable expectation to privacy solely hinges on the actual objective reality of the situation; instead there are also subjective considerations.
And even the definition of "objective" in "expectation to privacy" is what general society holds to be true -- it doesn't speak to demonstrating reality of privacy.
But then the 4h means nothing! Any communication that can technically be eavesdropped is "onbjectively" not private. So where is there a legal expectation of privacy?
It's completely based on precedent (i.e. court rulings). According to Wikipedia [http://en.wikipedia.org/wiki/Expectation_of_privacy] one appeals court has ruled that there is a reasonable expectation of privacy for e-mail, but that is something that the Supreme Court can overrule or not depending on what the Justices' personal take on the situation is.
