I sent a letter to someone and got it returned 2 weeks later. Stamped on the envelope is something saying "NOT DELIVERABLE AS ADDRESSED, UNABLE TO FORWARD". What's that mean?
They can't deliver it as you addressed it; there's probably no such street/house number/apartment. And there's no forwarding address on file for the _person_ you addressed it to so they can't forward it to their new address (if they've moved).