What is what to do when you get someone elses mail?

Here's what to do when you receive someone else's mail:

Don't Open It: This is the most important rule. Opening someone else's mail is a violation of federal law in many regions.
Mark it as "Misdelivered": Clearly write "Misdelivered," "Wrong Address," or "Return to Sender" on the envelope. This informs the postal service of the error.
Return to Mailbox: The easiest option is usually to put the mail back in your mailbox or a public mailbox. The postal carrier will pick it up and hopefully redirect it.
Hand it to a Postal Worker: If you see your mail carrier, you can hand them the misdelivered mail directly.
Drop it Off at the Post Office: You can also take the mail to your local post office and explain the situation.
Black out the barcode: Consider blurring out the barcode on the front. This helps prevent automated sorting machines from misrouting the mail again.
Don't Throw it Away: Discarding someone else's mail is also generally illegal.

Here are some important topics in the context of handling mail.