Are you saying that a letter was delivered properly, but the recipient wrote "No such number - return to sender" on it and gave it back to the PO? Or that the PO returned it without delivering it?
Either way, I can see it happening. Call the delivery PO and ask what was wrong with the address. If it checks out, it was probably a one-time fluke. If the recipient wrote return to sender on it, then ask if there's anything else that can be done.