Law Enforcement can also get access to that metadata -- it's called a mail cover. See U.S. v. Choate.
"A mail cover is a surveillance of an addressee's mail conducted by postal employees at the request of law enforcement officials. While not expressly permitted by federal statute, a mail cover is authorized by postal regulations in the interest of national security and crime prevention, and permits the recording of all information appearing on the outside cover of all classes of mail."
