In a decision ruled only yesterday, the United States Sixth Circuit Court of Appeals
has stated that e-mail should be treated like other private forms of communication when it comes to search and seizure. That is, the authorities have no more right to ask an internet service provider to turn over your e-mail records than they do to ask the post office to give them your physical mail, ask a phone service provider to tap your phones, or to enter your house without permission. In every case, they've got to get a warrant.
From the decision:
It only stands to reason that, if government agents compel an ISP to surrender the contents of a subscriber's emails, those agents have thereby conducted a Fourth Amendment search, which necessitates compliance with the warrant requirement.