In 2001, the Postal Service created the Mail Isolation and Control Tracking System (MICT) after several people died as the result of anthrax being sent through the mails to various targets.
This system allows USPS to retroactively track mail correspondence at the request of law enforcement. Although it was originally set up to improve the efficiency of mail sorting, it has become a valuable investigative tool. The FBI acknowledged the existence of the program in 2013 when investigating the poison-laced letters sent to the U.S. President, and Mayor of New York. The program is based upon images that are captured of all mail at more than 200 processing centers throughout the country. Supposedly the images that detail the outside of your mail are kept for a maximum of thirty days and then destroyed. Such information can provide names, addresses, return addresses and postmark locations. Whether the actual images are retained, or Optical Character Recognition is employed to translate the information to a data base is unknown but certainly could be easily accomplished.