It may be possible that your PC/network has been infected with a virus and used to send spam without your knowledge. The spam appears to come from your IP address and is therefore blacklisted. These Open-relay blacklists are based on open ports through which unauthorised network traffic is allowed to flow. Open-relay blacklists are the most widely used. Many of the better blacklists will run an automated script to verify that the evidence against you is genuine before blacklisting your IP.

A blacklist organisation will also list much larger blocks of IP addresses than those owned by the suspected abuser. So, if you are provided with an IP address and the spammer owns an address that is close in range to yours and the spammer gets listed on this type of blacklist, your IP address might be listed as well. Usually the reasoning behind this practice is that, by punishing innocent parties, the blacklister is putting more pressure on the ISP to disconnect the suspected spammer's Internet access.

Keywords; IP blacklist,PC/network, virus , spam ,knowledge,IP address, network traffic , spammer,ISP, Internet access