Two years ago, I did a post noting that the 6th Circuit backed off from an earlier decision in which it held that individuals have a 4th Amendment expectation of privacy in the contents of emails they leave stored with an ISP, such as Yahoo! As I noted in that post, the court didn't reject that proposition; it merely held that the decision reaching that issue was premature.
The 6th Circuit just issued a new opinion, one in which it properly addresses that issue and in which it holds that individuals have a 4th Amendment expectation of privacy in stored emails. I'll probably do a post on it later, when I've had time to read and digest the opinion.
In the meantime, you can find it here, if you're interested.