This seems odd, especially due to the financial woes of the United States Postal Service, but the "U.S. Postal Service photocopies the envelopes of all mail sent in this country, the New York Times reports."
What isn't known is how long the government saves the images of the 160 billion or so pieces of mail sent annually, or exactly how they are stored and used. But scanning 160 billion images and the time and additional manpower needed to store and retrieve those documents has to be prohibitively expensive -- not to mention an invasion of privacy.
"The Mail Isolation Control and Tracking program had been secret, but was revealed by the FBI last month in discussing an investigation of ricin-tainted letters reportedly sent to President Barack Obama and New York City's mayor, among others."
“In the past, mail covers were used when you had a reason to suspect someone of a crime,” Mark D. Rasch told the newspaper. He previously served as director of the federal Justice Department’s computer crime unit.
“Now it seems to be ‘Let’s record everyone’s mail so in the future we might go back and see who you were communicating with.’ " Rasch said. "Essentially, you’ve added mail covers on millions of Americans.”
The USPS is a government entity, so I'm not sure what I expected in terms of privacy. It does seem, however, that no communication is completely free from government intrusion these days.
ABA Journal -- Post office photocopies envelopes of all mail sent in the US, says NYTimes