To obtain electronic communications, the government must obtain a warrant for any that are held for 180 days or fewer by a computer service provider. This means establishing probable cause that the evidence sought is related to a crime.
But for anything older than that, investigators need only a grand jury or administrative subpoena, as long as the person whose communications are sought is informed. That notification can be delayed by as much as 90 days if disclosure might have an adverse effect, such as destroying or tampering with evidence.
Back in 1986, Congress viewed communications over six months old to be abandoned and therefore subject to reduced protection, a notion that looks quaint today when emails and texts may be held for years.
Another provision of the statute allows investigators to obtain information from the provider about a subscriber to any electronic service, like cellphones, by seeking a court order based on “reasonable grounds to believe” that the records are relevant to a criminal investigation. This is a lower standard than probable cause, the usual requirement for a search warrant.
It is this lower threshold for getting information that is at issue in Carpenter v. United States, which the Supreme Court will hear in its next term starting in October.
https://www.nytimes.com/2017/07/10/business/dealbook/digital-privacy-supreme-court.html