they have fabelsources registered with them under the list of blacklists to check for bad IP addresses. When an IP is blacklisted like this, it generally means the computer with the IP in question (which I will call, the machine from now on) is a source of sending spam.
For example, a program that continuously sends garbage emails to mass amount of random users was installed and currently running on the machine.
I agree mostly with Simon that it has no effect on SEO. It will have an effect if the spam actions consume enough resources on the machine to the point where a simple request to a web page on the same server takes significantly longer than usual. (for example 1 to 2 seconds to process a simple web page as opposed to the google recommended loading time of under 1/5 of a second).
If you're in this situation, upgrading the system by adding more memory or more CPU power is a temporary fix, but the permanent fix is to identify and remove processes that eat up all the CPU power and/or consume unusually large amounts of memory as well as blocking access to resources from IP addresses that are known to consistently abuse the machine.