The Sixth Circuit Court of Appeals ruled yesterday, in Warshak v. U.S., that people have a reasonable expectation of privacy in their email, so that the government needs a search warrant or similar process to access it. The Court’s decision was swayed by amicus briefs submitted by EFF and a group of law professors.

When Alice sends an email to Bob, the email will be stored, for a while at least, on an email server run by Bob’s email provider. Depending on how Bob uses email, the message may sit on the server just until Bob’s computer picks up mail (which happens every few minutes when Bob is online), or Bob may store his long-term email archive on the server. Either way the server, which is typically run by Bob’s ISP, will have a copy of the email and will have the ability to access its contents.

The key question in Warshak was whether, notwithstanding the ISP’s ability to read his mail, Bob still has a reasonable expectation of privacy in the email. This matters because certain Fourth Amendment protections apply where there is a reasonable expectation of privacy. The government had used a certain kind of order authorized by the Stored Communications Act to compel Warshak’s ISP to turn over Warshak’s email without notifying Warshak. Warshak argued that that was improper and the government should have been required to get a search warrant.

The key to the Court’s ruling is an analogy, offered by the amici, between email and phone calls. The phone company has the ability to listen to your calls, but courts ruled long ago that there is a reasonable expectation of privacy in the content of phone calls, so that the government cannot eavesdrop on the content of calls without a warrant. The Court accepted that email is like a phone call, for privacy purposes at least, and the ruling essentially followed from this analogy.

This is not a general ruling that warrants are required to access electronic records held by third parties. The Court’s reasoning depended on the particular attributes of email, and even on the way these particular ISPs handled email. If the ISP’s employees regularly looked at customer email in the ordinary course of business, or if there was a written agreement giving the ISP broad latitude to look at email, the Court might have found differently. Warshak had a reasonable expectation of privacy in his email, but you might not. (Randy Picker has an interesting commentary on Warshak in relation to online records held by third parties.)

Interestingly, the Court drew a line between inspection of email by computer programs, such as virus or spam checkers, versus inspection by a person. The Court found that automated analysis of email did not erode the reasonable expectation of privacy, but routine manual inspection of email would erode it.

Pragmatically, a ruling like this is only possible because email has become a routine part of life for so many people. The analogy to phone calls, and the unquestioned assumption that people value the privacy of email, are both easy for judges who have gotten used to the idea of email. Ten years ago this could not have happened. Ten years from now it will seem obvious.

Orin Kerr, who is expert in this area of the law, thinks this ruling is at higher than usual risk of being invalidated on appeal. That may be the case. But it seems to me that the long-term trend is toward treating email like phone calls, because that is how people think of it. The government may win this battle on appeal, but they’re likely to lose this point in the long run.