The late-2006 appearance of durable botnets was a tipping point in the back-and-forth battle against spammers, an industry analyst said Friday, who predicted that spam will continue to gain ground on defenses.Assembled by a Trojan called SpamThru, the new botnets are tougher to bring down, says Paul Wood, senior analyst with MessageLabs, a message security and filtering service. "The advent of Trojans like SpamThru makes it possible for each bot in the net to learn about the location of other bots.    more....