[W]e have little difficulty agreeing with the district court that individuals maintain a reasonable expectation of privacy in e-mails that are stored with, or sent or received through, a commercial ISP. The content of e-mail is something that the user “seeks to preserve as private,” and therefore “may be constitutionally protected.” Katz, 389 U.S. at 351. It goes without saying that like the telephone earlier in our history, e-mail is an ever-increasing mode of private communication, and protecting shared communications through this medium is as important to Fourth Amendment principles today as protecting telephone conversations has been in the past. See Katz, 389 U.S. at 352 (“To read the Constitution more narrowly is to ignore the vital role that the public telephone has come to play in private communication.”)