@book { DirektmarketingimInternetunddieSpamProblematik,
    title = "Direktmarketing im Internet und die Spam-Problematik",
    author = "Michael {Oeltjebruns}",
    year = "2004",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832479374",
    url = "https://m.diplom.de/document/223182"
}