Re:Sendmail (Score 1)

Why? Simple. They start to spam from their fixed location, and they get blackholed just as quick. Then they have to move to another IP. Which is a pain.

Better to fire your spam off through open relays (preferably anonymizing the true source), or through open http/socks proxies (inherently anonymizing), and just move to the next when you think the open relay has been burned.

There are spammer-providers who sell lists of open relays/socks/http proxies to spammers.