These lines have also been cited as dated, because zip codes have replaced zone numbers, and for that matter, I haven't know of the U.S. postal service returning anything to sender in quite some years. however, I do remember how years ago letters were sometimes returned with a stamp (ink stamp, that is) applied to them with blanks to check the reason for the mail being returned. one possible reason that I saw so indicated was "addressee unknown". I don't remember any such category as "address unknown". that makes sense, for as long as an address were unknown, how would one be motivated to even try sending the letter? so the "address unknown" reference in the song is in all likelihood a confused impression of the "addressee unknown" category.