At the request of law enforcement officials, postal workers record information from the outside of letters and parcels before they are delivered. (Actually opening the mail requires a warrant.) The information is sent to whatever law enforcement agency asked for it. Tens of thousands of pieces of mail each year undergo this scrutiny.
The Mail Isolation Control and Tracking program was created after the anthrax attacks in late 2001 that killed five people, including two postal workers. Highly secret, it seeped into public view last month when the F.B.I. cited it in its investigation of ricin-laced letters sent to President Obama and Mayor Michael R. Bloomberg. It enables the Postal Service to retroactively track mail correspondence at the request of law enforcement. No one disputes that it is sweeping.