When Congress passed the 1986 Electronic Communications Privacy Act (ECPA), a time when massive online storage of e-mail was essentially unimaginable, it was presumed that if you hadn’t actually bothered to download your e-mail, it could be considered "abandoned" after 180 days. By that logic, law enforcement would not need a warrant to go to the e-mail provider or ISP to get the messages that are older than 180 days; police only need to show that they have "reasonable grounds to believe" the information gathered would be useful in an investigation.
Senator Patrick Leahy, one of the ECPA's authors, wants to revise that law to keep up with the change in the basics of email -- most people don't use offline email programs anymore, so all our communication is, legally, accessible without a warrant. But his proposal, which "will be incorporated into a larger bill aimed at revising the 1988 Video Privacy Protection Act (VPPA)," would amend that problem.
The Ars Technica article I've linked and quoted points out that many tech companies have already taken to demanding warrants to protect their customers' data. Gmail, for example, is as protected as it ought to be. But not all companies are that ethical, and none of them are required.
Leahy is also selling the amendment on the basis that it's still leaving a lot of holes in citizens' protection -- all the meta data attached to an email is still available to the police. This is to sell the bill to the conservatives who tend to vote against anything that makes the police's job harder.
Political battles like this always make me nervous, because it's disappointing to see that we still have arguments in congress about whether it's a good idea to enforce the spirit of privacy and to protect the citizens from their government. But if I wanted to come at this from a starting position of pessimism, it's at least good that there are people fighting to keep the law up to date with the world it's legislating.