Jdog - try cloudmark, seems a great outlook add-on. I added it last week as the wifes been moaning like hell about spam .... it's keeping her quiet so it must be good.
They are testing to see if the email account exists. The messages should read "Congratulations. If you read this your email account has been upgraded to Tier II SPAM. Please provide adequate room for email storage and bandwidth to avoid any interruption of our preferred service." :rasta:
The spammers are quite smart and keep changing thier tactics. Now they send mail with RE: in the header to make you think they are replying to you as well as putting gibberish in the header to make you click out of curiosity.