Efficient event delivery in a content-based publish/subscribe system has been a challenging problem. Existing group communication solutions, such as IP multicast or application-level multicast techniques, are not readily applicable due to the highly heterogeneous communication pattern in such systems. We first explore the design space of event routing strategies for content-based publish/subscribe systems. Two major existing approaches are studied: filter-hosed approach, which performs content-based filtering on intermediate routing servers to dynamically guide routing decisions, and multicast-based approach, which delivers events through a few high-quality multicast groups that are pre-constructed to approximately match user interests. These approaches have different trade-offs in the routing quality achieved and the implementation cost and system load generated. We then present a new routing scheme called Kyra that carefully balance these trade-offs. Kyra combines the advantages of content-based filtering and event-space partitioning in the existing approaches to achieve better overall routing efficiency. We use detailed simulations to evaluate Kyra and compare it with existing approaches. The results demonstrate the effectiveness of Kyra in achieving high network efficiency, reducing implementation cost and balancing system load across the publish-subscribe service network.