We often take the postal service for granted, or even view it with annoyance for filling our mailboxes with loads of advertisements that we'll never read. However, the postal service is still an important institution, one with a surprising amount of power as a federal entity. To this end, crimes involving the postal service are generally federal offenses. Only in some certain circumstances is opening someone else's mail acceptable.
One such instance might be if you are staying in someone else's home and have their expressed permission to open their mail. In this instance, even though you are technically opening the mail of another person willfully, it is probably not a prosecutable offense. It is very important to make sure that you have the permission of the rightful owner of the mail.
Similarly, if you open someone else's mail accidentally, this is unlikely to lead to serious consequences. While it is illegal to open another person's mail, we all make mistakes. It is far more important to follow up this mistake by attempting to make it right.
One area that may cause particular difficulty is if you take someone else's mail and open it. It is actually a federal crime to remove someone's else's mail from a mailbox that does not belong to you without their permission. In a worst-case-scenario, this simple act could cost you several years in prison.
If you have concerns about the repercussions of a mail mishap, it is probably wise to speak with a lawyer directly. An experienced lawyer can help you examine your circumstances and craft a strong defense of your rights and freedoms.
Source: Findlaw, "Is It Illegal to Open Someone Else's Mail?," accessed Sep. 08, 2017