Practical applications:

Reading someone’s email or messages [SMS/WhatsApp]:

Just as it is forbidden to read physical mail [i.e. snail mail] addressed to another person, so too it is forbidden to read electronic mail [i.e. email], as the prohibition was against reading information, and invasion of privacy, and not necessarily against the physical handling of another person’s mail.

Accidently received another person’s mail

One who received another person’s mail may not open it to read. He is to notify the individual who it was intended for, or simply write “Return to Sender” or “Wrong Address” on the mail not addressed to you and put it in your nearest mailbox.

Investigative work for the sake of a Mitzvah or criminal investigation:[6]

It is permitted for a Beis Din, school, or parent, to look into another’s mail or private documents for the sake of their Torah education. Certainly, it is permitted to do so to prevent a crime and protect oneself and others from possible monetary loss or physical damage.

May one who received a letter or email from an individual reveal to others what was written?[7]

One who received a private letter or email from an individual is prohibited from sharing it with others, unless he receives explicit permission from the sender. Accordingly, that which people are accustomed to forward private emails or messages to other acquaintances, may only be done if one has explicit permission from the sender.

Withholding someone’s physical mail [snail mail]: Under U.S. federal law[8], it is illegal to intentionally stop a letter from being delivered to its intended recipient, and that may include not informing the U.S. Postal Service that you have another person’s mail. If a person snatches a letter that isn’t addressed to them and opens it, they are committing mail theft. Intentionally doing so is a federal crime that could potentially land one in prison for up to five years.

Reading someone’s mail or email: The Fourth Amendment to the United States Constitution provides a privacy protection in the sense that “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated.” The act of simply accessing and reading someone’s mail or email, without their permission can be considered a State or Federal crime, with civil and/or criminal liability, depending on the circumstances.[9] [A major difference however in the secular law, versus Takanas Rabbeinu Gershom, is that in the secular law, it is not illegal to read a piece of mail that was left open, while according to Rabbeinu Gershom, doing so is not allowed.]