In the case of Warshak v United States an appeals court has ruled that government entities must first obtain a search warrant before they are allowed to seize and search your email. This gives email the same privacy a letter sent through the post office or phone call would have.
The lawsuit, which started in 2006, when Steven Warshak filed a lawsuit alleging the government violated by his Fourth Amendment rights and certain provisions in the Stored Communications Act. According to the Electronic Frontier Foundation, the court felt that email should be protected under the Fourth Amendment, but the case was dismissed on procedural grounds. An appeal made by Warshak brought the case back to the court room, and the judges have once again agreed that email should be have the same privacy protections that other communication methods have.
The court held that,
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....
This is the only federal appeals court to have a ruling on the books giving email protections under the Fourth Amendment. This will put into question the legality of the Stored Communications Act which currently gives the government the authority to secretly look at your emails without a warrant in many situations.