The 6th Circuit Court of Appeals has issued an interesting opinion on email privacy. In doing so, it rules that a portion of the Stored Communications Act (SCA) violates the Fourth Amendment.
I’ll let the lawyers on the net provide a detailed explanation. As I understand the opinion, though, the question is what process, and hence what standard of proof, is necessary for the government to obtain access to stored email. The SCA sets out three mechanisms: a search warrant, a subpoena, or a court order. Search warrants, which do not require notice to the subject, require a high standard: "probable cause". The other two mechanisms have a lower threshold — relevance to an ongoing criminal investigation — but in general subjects can challenge such orders. In this case, the government relied on a provision in the SCA that allowed the government to delay notice, and hence forestall challenges. The Court struck that down. It said that the government can’t have it both ways: it can use the no-notice/ no-challenge search warrant, but with probable cause; or it can use the easier mechanisms, but only if the request can be challenged.
There are two other important points. First, the Court asserted that in general, users of commercial ISPs do have a legitimate expectation of privacy for their email. However, that expectation is dependent on the terms of service, which suggests that users really need to read those boring licenses carefully. Second, the Court distinguished between "technologies" that do some sorts of email scanning — anti-virus, anti-spam, etc. — and human examination of content. I suspect that that distinction will become increasingly fragile as technology improves.
The Court opinion is here.