Your phone calls aren't the only communications that the US government is collecting "metadata" on. For decades now, a snail mail surveillance system has allowed law enforcement agencies to log information that's displayed on the outside of your incoming and outgoing letters. This "mail covers" logging was originally limited to specific requests, but following the ricin scares in 2001, the system morphed into something massive: a full-blown surveillance program that scans the outside of every envelope and parcel processed by the US Postal Service, according to The New York Times.
The NSA's broad surveillance efforts have sometimes been justified by the legality of this very system, known as the Mail Isolation Control and Tracking program. The Times has a detailed history of how the program came to be, covering the arrests made because of it, the minimal oversight that governs data requests, and just how long it's been since Congress considered the practice's legality.