After 180 days in the U.S., email messages lose their status as a protected communication under the Electronic Communications Privacy Act, and become just another database record. This means that a subpoena instead of a warrant is all that's needed to force Google to produce a copy. Other countries may even lack this basic protection, and Google's databases are distributed all over the world. Since the Patriot Act was passed, it's unclear whether this ECPA protection is worth much anymore in the U.S., or whether it even applies to email that originates from non-citizens in other countries.