Port 25 blocking spurred ISP email relay zombies

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“This represents a new step in the evolution of spambots. Previously, these bots all tried to transmit directly from port 25, but with the advent of port 25 blocking by ISPs, this has become an obstacle. It was only a matter of time before spambots began trying to relay through mail servers.”

- Edward Flak’s May 30, 2007 entry in The Spam Diaries

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