Gray Listing. What Is It?

I was manning the EEC booth at the OMMA conference in New York last week when I met an academic looking fellow named David Blumenstein.  After I pitched him about the EEC and all the cool things the group was working on he grabbed a seat next to me and started to rant about how gray listing is the secret to fighting spam.  Naturally the subject piqued my interest so I prodded him to go into detail.  He said that spammers don't like to receive bounces because it overwhelms their servers.  As a result spammers turn off bounce mechanisms.  Therefore pinging a legitimate mail server will yield a mailer demon and pinging a spammer's server will yield nothing.  He concluded that putting all unknown senders in email purgatory until a mailer demon is received helps to eliminate most spam.  The biggest drawback is that his email is delayed for 12 minutes or so. 

Pretty interesting!  Thanks Dave!  I'll have to try it out.