Given the fundamental similarities between email and traditional forms of communication [like postal mail and telephone calls], it would defy common sense to afford emails lesser Fourth Amendment protection…. 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…. [T]he police may not storm the post office and intercept a letter, and they are likewise forbidden from using the phone system to make a clandestine recording of a telephone call–unless they get a warrant, that is. 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….

***

If we accept that an email is analogous to a letter or a phone call, it is manifest that agents of the government cannot compel a commercial ISP to turn over the contents of an email without triggering the Fourth Amendment. An ISP is the intermediary that makes email communication possible. Emails must pass through an ISP’s servers to reach their intended recipient. Thus, the ISP is the functional equivalent of a post office or a telephone company. As we have discussed above, the police may not storm the post office and intercept a letter, and they are likewise forbidden from using the phone system to make a clandestine recording of a telephone call—unless they get a warrant, that is. See Jacobsen, 466 U.S. at 114; Katz, 389 U.S. at 353. 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 absent some exception.

– Judge Boggs, of the 6th Circuit Court of Appeals, ruling today that email users have a Fourth Amendment-protected reasonable expectation of privacy in the contents of their email accounts.  The EFF provided a critical amicus brief in the case, so please consider a year-end contribution to the EFF.

Update: More from Declan McCullagh at CNET:

Today’s decision striking down part of the 1986 Stored Communications Act rebuffs arguments made by the U.S. Department of Justice, which insisted the law was constitutional. In a brief (PDF) filed during an earlier phase of the case, prosecutors argued that the Fourth Amendment doesn’t apply because “compelled disclosure of e-mail is permissible under most providers’ terms of service.”

Since 1986, the general rule has been that police could obtain Americans’ e-mail messages up to 180 days old only with a warrant. Older messages, however, could be accessed with an administrative subpoena or what’s known as a 2703(d) order, both of which lack a warrant’s probable cause requirement.

Since then, the Sixth Circuit ruled, technological life has changed dramatically:

Since the advent of e-mail, 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 e-mail. Online purchases are often documented in e-mail accounts, and e-mail 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 e-mail account, as it provides an account of its owner’s life. By obtaining access to someone’s e-mail, government agents gain the ability to peer deeply into his activities.