Question: I am aware that there is a cherem (ban) instituted about a millennium ago by Rabbeinu Gershom against reading someone elses mail. But I have reason to believe that a neighbor of mine is planning to send a letter whose contents may unjustly harm me. Under such circumstances is it permissible for me to open that letter to see if it poses a danger to me?

Answer: Rabbeinu Gershom also instituted a cherem on polygamy and a funny story is connected to these two bans. At a military academy which had some married students living away from home on its campus, a Jewish secretary had instructions to open and screen the letters for reasons of national security. Upon opening one such letter he rushed to share its contents with its recipient.

"Mazal tov," he excitedly informed him, "Your wife had a boy!"

"Which one of my wives?" he asked.

"You mean to say that you violated the cherem of Rabbeinu Gershom by taking more than one wife?" was the incredulous response.

"And did you not violate the cherem on reading anothers mail?"

From an ethical point of view it is reasonable to assume that the cherem against reading anothers mail was never intended to protect the privacy of someone who intends to exploit it in order to unjustly harm another. Legally speaking, writes one of the great halachic authorities, the ban against reading anothers mail is based on the principle that borrowing the property of another without his permission is tantamount to theft. It follows then that just as someone may enter anothers property without his permission in order to recover an item which belongs to him he may also invade the privacy of a letter which may cause him damage.