A botnet which experts believe sent out 18% of the world's spam email has been shut down, a security firm said.Grum's control servers were mainly based in Panama, Russia and Ukraine. Security company FireEye and spam-tracking service SpamHaus worked with local internet service providers (ISPs) to shut down the illegal network. A botnet is a network of computers that has been hijacked by cybercriminals, usually by using malware. "Grum's takedown resulted from the efforts of many individuals," wrote Atif Mushtaq, a security researcher with FireEye. "This collaboration is sending a strong message to all the spammers: Stop sending us...