In U.S. v. Warshak, the United States Court of Appeals for the Sixth Circuit held that email correspondence enjoys substantially the same expectations of privacy and therefore of Fourth Amendment protections as do snail mail and telephone conversations. Warshak was a criminal case involving many millions of dollars and substantial fraud against numerous customers. Electronic communications, including some twenty-seven thousand emails, were important to the prosecution’s case.

The court held that:

The government may not compel a commercial ISP [internet service provider] to turn over the contents of a subscriber’s emails without first obtaining a warrant based on probable cause. Therefore, because they did not obtain a warrant, the government agents violated the Fourth Amendment when they obtained the contents of Warshak’s emails. Moreover, to the extent that the SCA [Stored Communications Act] purports to permit the government to obtain such emails warrantlessly, the SCA is unconstitutional. (emphasis added)

Two essential tests were employed in determining Fourth Amendment applicability. First, whether the criminal defendant had a reasonable expectation of privacy, and second, whether society in general recognizes one. The court found:

Given the often sensitive and sometimes damning substance of his emails, we think it highly unlikely that Warshak expected them to be made public, for people seldom unfurl their dirty laundry in plain view.

As to the second test, the court found that:

This question is one of grave import and enduring consequence, given the prominent role that email has assumed in modern communication. Cf. Katz, 389 U.S. at 352 (suggesting that the Constitution must be read to account for “the vital role that the public telephone has come to play in private communication”). Since the advent of email, the telephone call and the letter have waned in importance, and an explosion of Internet-based communication has taken place. People are now able to send sensitive and intimate information, instantaneously, to friends, family, and colleagues half a world away. Lovers exchange sweet nothings, and businessmen swap ambitious plans, all with the click of a mouse button. Commerce has also taken hold in email. Online purchases are often documented in email accounts, and email is frequently used to remind patients and clients of imminent appointments. In short, “account” is an apt word for the conglomeration of stored messages that comprises an email account, as it provides an account of its owner’s life. By obtaining access to someone’s email, government agents gain the ability to peer deeply into his activities.

The court drew a further analogy to telephone conversations, noting that they are “fully protected by the Fourth and Fourteenth Amendments” even though telephone companies have the capability to monitor and record calls. The court drew a similar analogy to snail mail, stating:

While a letter is in the mail, the police may not intercept it and examine its contents unless they first obtain a warrant based on probable cause. This is true despite the fact that sealed letters are handed over to perhaps dozens of mail carriers, any one of whom could tear open the thin paper envelopes that separate the private words from the world outside. Put another way, trusting a letter to an intermediary does not necessarily defeat a reasonable expectation that the letter will remain private.

Given the fundamental similarities between email and traditional forms of communication, it would defy common sense to afford emails lesser Fourth Amendment protection. … Over the last decade, email has become “so pervasive that some persons may consider [it] to be [an] essential means or necessary instrument for self-expression, even self-identification.” It follows that email requires strong protection under the Fourth Amendment; otherwise, the Fourth Amendment would prove an ineffective guardian of private communication, an essential purpose it has long been recognized to serve. (Internal citations omitted)

It is noted here that:

Today’s decision is the only federal appellate decision currently on the books that squarely rules on this critically important privacy issue, an issue made all the more important by the fact that current federal law — in particular, the Stored Communications Act — allows the government to secretly obtain emails without a warrant in many situations.

However, it wasn’t quite that simple in the Warshak case nor may it be in subsequent cases in different circuits.