Consider this, if all of you communication is done through letters and snail mail, then no government entity has the right to look through them without a warrant. This inalienable right is protected by the Fourth Amendment as detailed below:
" The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized"
But, in today's modern digital age, most communication is via email, text messages, and other electronic methods. These mediums are subject to monitoring without a court order for at least seven days prior to making an official request for permission to monitor them. Also, if your email sits on a server for longer than 180 days, it is considered abandoned and legal for any government official to look through without a warrant or court order.
You can find the text of the Constitution online at:
Third Amendment specifically: