Yes, in fact they do: http://www.nytimes.com/2013/07/04/us/monitoring-of-snail-mail.html http://www.nytimes.com/2013/07/04/us/monitoring-of-snail-mai... Though

Yes, in fact they do: http://www.nytimes.com/2013/07/04/us/monitoring-of-snail-mail.html http://www.nytimes.com/2013/07/04/us/monitoring-of-snail-mai... Though I'm sure the primary purpose of scanning envelopes is, ya know, to see where they're going.