The unwanted messages and advertising, probably from "Spamers," are sent
out to addresses obtained when they (the Spamers) get a reply resulting
from one of their probes. Until they receive an answer they don't know
if an address is good or not. Once they have a reply and a known address
they are home free and there's little you can do to stop their mailings.
It's a problem being studied by several different factions, including
the Feds. The best way to handle Spam mail and still have the freedom of
the net we now enjoy is to just ignore the "junk" mail... and don't
reply to it.