The government got Warshak's ISP to turn over all his emails, past and all future ones, without getting a search warrant. The government relied upon the Stored Communications's Act, which when passed in 1986, had not contemplated email and the public's utter reliance upon it. The 6th Circuit found email analogous to regular letters and said, "As we have discussed above, the police may not storm the postoffice 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." But since the SCA was there, the government relied upon it in good faith and thus the emails were not supressed.
One would hope that going forward, the government, in whatever manifestation it appears in, would have to get a search warrant to open mail, electronic or otherwise.