This is going to be an interesting one.
In a drugs case, Yahoo has been called to explain how supposedly deleted e-mails were recovered. It'll be embarrassing if there's a backup. However, the speculation of a surveillance program might be the answer.

Defense lawyers in the case claim that six months of deleted emails were recovered—something which Yahoo's policies state is not possible. The defense therefore speculates that the emails may have instead been collected by real-time interception or an NSA surveillance program. U.S. Judge Wants to Know How "Deleted" Yahoo e-Mails Were Recovered [motherboard.vice.com]