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.
Those are all mechanisms, not legal protections. All could be bypassed by a determined person.
In both email and physical mail, someone else can secretly read your mail if they try hard enough. In both, you expect them not to. In both, we have the same question: should the government need a warrant to violate that expectation?
I can plant a secret microphone in your house. That doesn't mean I have a right to.
http://about.usps.com/who-we-are/privacy-policy/intelligent-...
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-...