From: "Frank Haber" 

>The way it works is you run a blacklist, when a blacklisted spambot tries to
connect to your MX it gets immediately dropped, so naturally it now tries
to connect to the servers listed as the other MX records, and since there
isn't anything there they wait for a timeout, then try the third MX and
again wait for a timeout, then the next bot in the botnet tries to deliver
the spam and goes thru the same routine.

Neat hack, but that's one of the saddest things I've read recently. 
Remember when we once had an Internet?

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