The 4th Amendment was written for situations such as these. While the internet is used by the general public, the intent of a private email address is to keep that correspondence private and secure. People treat email with the same regard they have for normal postage. When I use my email, I don't reasonably expect the public to have access to the messages I am sending, and I am outraged if someone opens it up and looks at it without my authorization. With that mindset, I say that a warrant should be obtained before searching someone's private correspondence. To simply rummage through someone's mail without a warrant violates that person's constitutional rights to privacy as well as due process.