What is the difference between junk mail and unsolicited mail?

Even though I have a sign on my letterbox saying NO JUNK MAIL I regularly find junk mail stuffed in to my letterbox. When I asked a deliverer why he disregarded my request for no junk mail, I was told it was not junk mail, but "unsolicited" mail. I'm not aware of any difference. Shall I have to get another sign saying NO UNSOLICITED MAIL as well?