Tampering with mail is a crime and can be punishable by incarceration, fines, or probation. The United States federal government defines the act as "mail fraud," which carries severe penalties such as imprisonment time in prison depending on your location within America's borders.
Mail is considered private property under the laws in most states which means that if you expect privacy when it comes to your mail, then any person who breaches this will be guilty.
State laws differ, but in most cases, it is illegal to open someone else's mail. The only exception comes with postal workers or other carriers who are in good faith when opening your addressed letters for you as part of their job responsibilities. Though even then, there are restrictions about what they can take out if the sender did not allow this kind of access beforehand!
Mail tampering is one of the most commonly committed crimes, but it can also happen when you remove your neighbor's mail or throw away important documents that were meant for someone else. Mail theft often leads to more serious issues such as identity theft.
Mail fraud is a crime that involves mail, but unlike tampering, it's federal. Mail Fraud can be used to carry out schemes or plans to unlawfully get money or anything else of value through United States Postal Service (USPS) facilities in order for their scheme not only to violate state law but also become punishable by both state and federal penalties if committed so maliciously with intent to harm another person/businesses livelihoods, etc.