Postal inquiry

Question: If you get a piece of mail addressed to someone who no longer lives at your address, and it looks like it's important, and you don't know their forwarding address, are you actually allowed to do something along the lines of writing "Return to Sender" and an explanation on the envelope and putting it back in the mailbox? I've never tried anything like that and it seems iffy, but I've got a letter from the San Francisco traffic court here addressed to some guy I've never heard of and I'd like them to know he never got it so he doesn't get in trouble for missing a court date or something.