Mailboxes are considered federal property, and it's against the law to vandalize a mailbox or to open any mail deposited in it before those items are delivered to the addressee. There are no specific rules about opening someone's mailbox when you don't intend to open or destroy the letters. Still, it's wise to get the owner's permission before you wind up in court accused of mail theft.
Generally, yes, it's illegal to open someone's mailbox when your intention is to open or destroy the mail. The only exceptions are when you have the owner's permission, or you are handling the affairs of a person who died.
Snatching Someone's Mail is a Federal Crime
While there are no specific rules regarding opening a mailbox, snatching a letter from someplace other than your own mailbox is a federal crime. If prosecuted for mail theft, you face fines of up to $250,000 and five years in federal prison. There's also a federal law which prohibits the "Obstruction of Correspondence." Under this law, anybody who takes a letter to hide it, destroy it or pry into the business or secrets of another person – even if they don't open the letter – can be fined or imprisoned for up to five years, or both.
When You Have the Owner's Permission
If you're looking after someone's affairs while she's away, and the person has expressly asked that you take care of her mail, then there's no need to worry. To be prosecuted of a federal mail theft crime, there must be an element of fraud or deception. Simply looking after your neighbor's mail – being a good citizen – is unlikely to land you in court on federal mail theft charges.
When Someone Has Died
When you know that someone has died and his mail is piling up, you may open and manage the mail in certain circumstances. You'll have to show the United States Postal Service that you're the deceased person's executor or administrator, and complete a change of address form at your local post office. This will redirect the deceased person's mail to your own address, and you can manage the mail from there. Don't be tempted to break into the deceased person's mailbox even if you are the legal executor. There's a risk you may be committing the crime of letter box destruction, which carries a maximum prison sentence of three years.
Entering a Neighbor's Property May Be Trespassing
Trespass covers a much broader range of actions than most people realize and simply entering onto someone's property to access their mailbox could land you in hot water if the person doesn't want you there. Pay attention when the property owner tells you not to go on the property, when the property is fenced or gated, or when a "no trespassing" sign is posted. These are all signs that you don't have the owner's permission to enter her property, including the area where she keeps her mailbox.