This paper presents a 'Spam Zombie Detection' system which is an online system over the network that detects the spam and the sender of the spam (zombie) before the receiver receives it. Thus all the detection work is done at sender level itself. This paper focuses on a powerful statistical tool called Sequential Probability Ratio Test, which has bounded false positive and false negative error rates on which the Spam Zombie Detection system is based. This system is mainly implemented over the

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... ivate mailing system. It also provides the enhanced security mechanism in which, if the system which has been hacked i.e. it has become a zombie, then it gets blocked within the network.