Flickr User: Julio Martinez
Violation of one's privacy isn't really a tricky subject. If it is password-protected and you don't have permission to be viewing their computer/email/messages then that's morally wrong, but in South Carolina it seems that it isn't legally wrong.
The case the state Supreme Court ruled on involved a man's wife who asked her daughter-in-law to get into her husband's email and find out his mistress' name. According to The Sun News, while the state Supreme Court ruled in favor of the law and the wife, they were not too happy with the wife and in no way, "condoned her behavior".
To be fair, this isn't a "South Carolina courts is out of touch" piece, but more of a "most online law/privacy legislation is out of touch". The state Supreme Court followed and interpreted the "Stored Communications Act" as it was written. However, the problem is that the law was written in 1986, back when email was but a baby and the common man didn't really have an email address or, for that matter, fully understand what the whole "Wold Wide Web" thing was anyway.
Basically, the Stored Communications Act says that online, third party services (like email) are not protected under the 4th Amendment of the Constitution.
...Fourth Amendment defines the "right to be secure" in spatial terms that do not directly apply to the "reasonable expectation of privacy" in an online context. In addition, society has not reached clear consensus over expectations of privacy in terms of more modern (and developing, future) forms of recorded and/or transmitted information.
Furthermore, users generally entrust the security of online information to a third party, an ISP. In many cases, Fourth Amendment doctrine has held that, in so doing, users relinquish any expectation of privacy.
The law is very hazy. If the mail is on a third party service, like Google, then it's not violating one's privacy; however, if the mail had been downloaded onto a laptop then it could be violating privacy. The Sun News reports that the judges focused most of their attention over a portion of the Act that states, “Electronic mail is a form of communication by which private correspondence is transmitted over public and private telephone lines … if the addressee is not a subscriber to the service, the electronic mail company can put the message onto paper and then deposit it in the normal postal system.” We can't help but wonder if the question was asked, "Can it be proven that the 'electronic mail' was transmitted over telephone lines?" really, who uses dial up these days?
So in short, while it might be morally wrong for that jerk in the office to look through your email, he (or she) isn't breaking any laws.