Earlier quoted context omitted.
Uhh... In the postal system, your message is generally encapsulated in a tamper-evident envelope, carried in locked cars and trucks that enjoy Federal protection against intrusion, and end up in mailboxes that are almost always on private property and/or locked.
You can unglue an envelope over steam or on a hot surface. That's beside the point though. People in general expect their mail correspondence to remain private, although a motivated third party might be able to defeat security.
http://about.usps.com/who-we-are/privacy-policy/intelligent-...