You can return to sender mail that is addressed to someone not living at your residence and you can return junk mail as well (see the wikiHow How to Stop Junk Mail by Sending it Back ). If the mail is addressed to you personally, you can still return to sender if you don’t want it but consider the possible consequences first before doing so. For example, if it’s a bill, returning to sender doesn’t remove the necessity that you pay for it. And if you’re trying to send a message to someone whose messages you’re never going to read again (like someone you’ve broken up with), this may not stop them from sending mail to you and you may need to find a more direct way to get them to stop bothering you.