Wis. Stat. § 942.05: Opening letters.

Whoever does either of the following is guilty of a Class A misdemeanor:

(1) Knowing that he or she does not have the consent of either the sender or the addressee, intentionally opens any sealed letter or package addressed to another; or

(2) Knowing that a sealed letter or package has been opened without the consent of either the sender or addressee, intentionally publishes any of the contents thereof.