When you go to check your mailbox, whether at work or at home, you might come across something that is addressed to another person. You should always remember to send those items back to the sender or to give the mail to the person listed on the address label. Failing to do so could land you in serious trouble.
It isn't likely that a United States Post Office employee is monitoring your every move to determine if you are opening mail that isn't your own. There are still some instances in which you might face federal criminal charges if it is found out that you are opening mail that belongs to someone else.
The issue of opening someone else's mail isn't illegal if you had that person's permission. It also isn't illegal if you honestly didn't know that the mail belonged to someone else. In these cases, you likely won't face legal troubles.
If you do happen to open another person's mail without realizing that it wasn't your own, you should make every effort possible to let that person know what happened. This could mean returning it to the mail person or taking it to the person. One of the worst things you could do is try to hide the fact that you got the mail, especially if it was something that is important or that can be tracked.
When criminal charges do come as a result of you opening someone's mail, you must realize that you are facing up to five years in prison for mail theft. This is a serious matter that must be addressed accordingly.
Source: FindLaw, "Is It Illegal to Open Someone Else's Mail?," Brett Snider, Esq., accessed Dec. 08, 2017