Absolutely. This is basic fourth amendment protection. Not only must there be a warrant, but it must be specific. There is no authority for anyone to search all my email for anything suspicious because they have a hunch. A constitutional warrant requires you say which emails (say, a specific range of dates), for what, and what evidence you have. That requirement of specificity is in the fourth amendment, too. Yes you have to get a warrant, and it has to be the right kind of warrant rather than one of these sweeping unconstitutional ones that the NSA gets.