According to a story in today’s New York Times, the US Postal Service has a program to photographed the exterior of every single piece of mail they processed — something like 160 billion pieces a year — and provides that data to “law enforcement” upon request. So if you were thinking to evade Prism by brushing up on your letter writing skills . . . return to sender.
The “Mail Isolation Control and Tracking” program simply provides the information available on “covers” without the necessity of recourse to a judge. The article notes that challenges to this practice have failed on the basis that there is no reasonable expectation of privacy with regard to the information exposed on the outside of envelopes or other mail wrapping. This, of course, is a different matter from a practice of routinely obtaining this information in a widespread fashion or “later use” or for big data analysis.