If it's not first class letter mail, it just gets tossed at the post office. You still get the satisfaction of knowing it has to go through processing again before they toss it. Wastes their time twice. Nowdays, if you don't write anything on it and don't cross out the barcodes, it could potentially come back to you forever. Some post offices are really bitchy and will pull tape off of envelopes to see the address underneath, even if you stamp "RETURN TO SENDER" on it.