Spamming on shared hosting

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http://forums.cpanel.net – Hi, We have a whm/cpanel web hosting server which hosts many websites. All websites shared 01 IP address. Customer A has a hosting plan with us which also has email hosting Recently Customer A's computer was effected with a trojan/virus started to send thousands of spam emails to outside and that made our shared IP black listed with several Spam blocking servers so they could not send emails to some users. This also effected all the other customers on our server as they also use the same IP to send emails. My questions are 1. Is this a normal situation for shared hosting? Or is there (HowTos)