The Electronic Communications Privacy Act says all email stored on a server that is older than 180 days is not protected under the 4th Amendment.
Furthermore, the IRS appears to operate under the assumption that no email is protected.
> New documents released to the ACLU under the Freedom of Information Act reveal that the IRS Criminal Tax Division has long taken the position that the IRS can read your emails without a warrant—a practice that one appeals court has said violates the Fourth Amendment (and we think most Americans would agree).
I've always wondered if I delete the email, does it count as "no longer stored"? I should, as a user, reasonably assume so, but I know it's kept longer than that, probably indefinitely.
If the law is that the NSA just has to request it, no warrant necessary, there you go.