IP-based blacklist is an effective way to filter spam emails. However, building and maintaining individual IP addresses in the blacklist is difficult, as new malicious hosts continuously appear and their IP addresses may also change over time. To mitigate this problem, researchers have proposed to replace individual IP addresses in the blacklist with IP clusters, e.g., BGP clusters. In this paper, we closely examine the accuracy of IP-cluster-based approaches to understand their effectiveness and fundamental limitations. Based on such understanding, we propose and implement a new clustering approach that considers both network origin and DNS information, and incorporate it with SpamAssassin, a popular spam filtering system widely used today. Applying our approach to a 7-month email trace collected at a large university department, we can reduce the false negative rate by 50% compared with directly applying various public IP-based blacklists without increasing the false positive rate. Furthermore, using honeypot email accounts and real user accounts, we show that our approach can capture 30% – 50% of the spam emails that slip through SpamAssassin today.