Most people understand that it's illegal to open mail that's not addressed to them. What's not so widely understood is just how serious the consequences can be. Intentionally opening, intercepting or hiding someone else's mail is the felony crime of mail theft. It comes with some heavyweight penalties, including five years' incarceration in a federal prison.

You should be okay if you open mail accidentally, however, as long as you take the proper steps to reunite the mail with its owner.

Doing anything at all to someone else's mail could land you in serious hot water, even if you just glance at the return address while handing it to them. A couple of federal laws make it illegal to take, steal, intercept, open, damage or destroy someone else's mail, or impede the delivery of mail. You could be looking at charges of mail theft or "obstruction of correspondence" if you're caught doing any of these things.

Both crimes carry fines of up to $250,000 and five years in a federal prison.

Read More: Is it Illegal to Open Someone's Mailbox?

Obviously, the law isn't going to punish you if you didn't know it was someone else's mail when you opened it. It's extremely easy to open a letter without paying attention to the addressee, especially if you're opening a big pile of mail and the letter was accidentally posted to the wrong address. It's what you do next that's important.

Toss the mail in the garbage and you're potentially obstructing the mail delivery. This might seem like the safest response – getting rid of the evidence – but it's a federal crime. The best thing to do is re-seal the letter, write "Return to Sender" on the envelope and pop it back in the mail.

You're not going to get in trouble by following her instructions if someone has asked you to open her mail. The U.S. Postal Service is chiefly concerned with items that are deliberately stolen from a carrier or a mailbox because these items can be used to commit other crimes, such as identity theft. The U.S.P.S. isn't going to care that you're opening a neighbor's mail while she's in hospital. As long as you have permission, it's highly unlikely you're going to wind up in court on federal mail theft charges.

The owner or tenant before you might continue to receive mail at your home if you just recently moved in. This is especially common if they didn't forward their mail with the U.S.P.S. or if they receive mail that services can't forward.

It's reasonable to assume that mail in your own mailbox is yours without looking at the name, so if you open the previous tenant's letter on accident, you might be off the hook. Knowingly and purposefully opening another person's mail is still illegal if the mail came to your address with someone else's name on it.

Don't discard letters or packages that come to your home addressed to the person who lived there before. Simply write, "Wrong Address" or "Return to Sender" on the envelope instead and give it back to the service that sent it. Redirecting the mail is the safest way to make sure you stay on the right side of the law.

It's not illegal to open someone's mail if the addressee has died and you're authorized to manage her estate. You'll have to fill out a redirection request at the post office in this situation, and the U.S.P.S. will make sure that the deceased's mail is delivered to your own address.

It's against the law to break into someone's mailbox even if you're legally permitted to read the mail – do this, and you could be charged with the crime of mailbox tampering.