Q. Since my business e mail address is similar to someone else, I sometimes get sent this individual’s e mails by mistake. (I know it is not for me because I have no dealings with the sender. I have also complained to the sender, and for a while it worked but then he does the same mistake again) In order to see if it is an important message, is it permitted to open it, and reply so the sender who would then forward the mail to the right address, or would that be included in the Charem of Rabennu Gershon?

A. At the end of the Responsa Maharam, on the list of the decrees instituted by Rabbeinu Gershom Meor Hagola, he includes the cherem or ban against reading other people’s mail. However he adds that if the letter was discarded, it is permissible (See also Kol Bo s. 116, Beer Hagola Y.D. 334). This cherem is still effective today, Poskim write that it should be considered with the same graveness as any other Torah prohibition. (Chikekei Lev Y.D. 49 debates whether three biblical prohibitions are involved). It was embraced and accepted at the time that it was issued by all Jewish communities throughout the world. No time limitation placed on this ban (See Shiltei Givorim – Shavuos c. 5, Chevel Nachalaso 84)
Horav Shlomo Miller’s Shlit”a opinion is that e mails are similar to any other letter and are included in the cherem, since people do mind if others read their private e mails. (A similar ruling is quoted in the name of Horav Chaim Pinchas Scheinberg zt”l at ohrsameach.org)
In this particular case, when the e mail was sent by mistake to the wrong recipient due to the similarity of the addresses, Horav Shlomo Miller’s Shlit”a opinion is that it may be correct for the erroneous receiver to open the e mail in order to reply to the sender and make him aware that it was sent to the wrong address if no other option is available. However, it would be better not to open the mail, and just send a new e mail to the sender, making him aware of the error done (the e mail would be identified by the date and time it was sent. Another possibility available in some servers, is to click on the adjacent box to the e mail and without opening, forward or return to sender)

Rabbi A. Bartfeld as revised by Horav Shlomo Millers Shlit”a