Someone or something on your network sent us spam. Some network (in particular in South America and Asia) don't maintain very good records in whois, so a large netblock including your IP might have been included in our list.
We send abuse complaints to the abuse contact listed in 'whois' for the IP address. If this information is incorrect or we don't get a reasonable response we're much more likely to list an IP block as being unwanted for sending mail to our network.
The list only includes IP addresses, not domain names.