@article{ duan07dmtp,
    author = "Zhenhai Duan and Yingfei Dong and Kartik Gopalan",
    title = "DMTP: Controlling Spam Through Message Delivery Differentiation",
    journal = "Computer Networks", 
    volume = "51", 
    number = "10",
    pages = "2616-2630",
    year = "July 2007"
}