Cherem d'rabbeinu gershom
Not Opening Other People's letters
It is circa 1000 CE, and sending letters to people in other cities and even countries has never been simpler. However, along with these amazing advances, comes a new potential problem, invasion of privacy. How can you trust the messengers not to read your mail?
Rabbeinu Gershom Meor Hagolah (the "Light of the Diaspora", 960 - 1040) knew that as the greatest Ashkenazi leader of his day, he had the power to reduce such breaches of privacy. He convened a synod to issue several amendments or takkanot to Jewish law. The most well-known is his ban on polygamy. Another takkanah prohibited reading other people's letters.
These takkanot were called charamot, bans. The penalty for violation was to be put in cherem, exclusion from the community and social ruin. However interesting the contents of other people's mail may have been, the spectre of exclusion was deterrent enough to create a relatively secure postal system. Until recently, some religious Jews would write over the seal of letters delivered by messenger an acronym for "Under the ban of Rabbeinu Gershom".
It goes without saying that hacking into other people's voicemail also comes under the ban.