Is it a federal offense to open someone's mail?

It is a federal crime for someone to intentionally open mail that belongs to another individual or business, according to TheLawDictionary.org. It is not considered a crime if someone accidentally opens a piece of mail addressed to someone else.

While the law allows for someone to mistakenly open a piece of mail that does not belong to him, it does not allow for that person to improperly dispose of that mail. The individual who mistakenly opened the mail must make every reasonable effort to ensure that piece of mail arrives in the hands of its rightful owner. The best thing someone can do in this circumstance is to return the mail to the care of the United States Postal Service.