There’s some ruling that any mail left on the server for more than 60 days or so is considered abandoned and is essentially an all-you-can-eat buffet for law enforcement, even if you search/read it every day.
> In enacting the ECPA, Congress concluded that customers may not retain a "reasonable expectation of privacy" in information sent to network providers. . . [I]f the contents of an unopened message are kept beyond six months or stored on behalf of the customer after the e-mail has been received or opened, it should be treated the same as a business record in the hands of a third party, such as an accountant or attorney. In that case, the government may subpoena the records from the third party without running afoul of either the Fourth or Fifth Amendment.
The third party isn't even obligated to let you know that the subpoena happened - Google usually does (unless there's a gag order), but does not promise to AFAIK, and I am not aware of the policies of other providers.