I worked for the Post Office for many years.
NO ONE is supposed to go into your mailbox
except the addressee or a postal official (letter
carrier, postal supervisor, etc.). Your mailbox is
owned by the US Post Office. It is a "little piece"
of government property attached to your house.
CALL the local POSTMASTER'S office who'll probably
contact the POSTAL INSPECTORS.
THIS ANSWER IS PURELY FOR ACADEMIC DISCUSSION ONLY AND DOES NOT CONSTITUTE ANY TYPE OF LEGAL ADVICE OR LEGAL REPRESENTATION.