A small padded envelope I sent to another trader is showing "Undeliverable as Addressed".  Anyone seen this before.  Before the tracking showed this it was "Out for Delivery" to the correct location.  Does this mean the mailmen just decided not to deliver the package?

I had it recently on a shipment to an apartment in SC. I informed buyer and she said she has never had a problem getting packages there before, but this was a pretty big box. Anyway I suggested she make a trip to the post office and check it out. Heard back about a week later that she picked it up there, but P.O. never updated the tracking, it still shows undelivered.