The FindLaw Blotter reports that intentionally opening anyone's mail or mailbox without their permission is a felony. However, if you have permission to check a neighbor's mail while they are away or intend to forward the contents of the mailbox, doing so is not a crime.
According to US Code, anyone who takes or opens another person's mail will either be fine or imprisoned. Mail theft can occur from a post office, private mail box or any other authorized receptacle. It is equally forbidden to tamper with someone's private mailbox as it is to tamper with a public mail receptacle. It is also illegal to buy or sell any item stolen from a mailbox.