Disclosed herein are filtering systems and methods that employ an electronic message source reputation system. The source reputation system maintains a pool of source Internet Protocol (IP) address information, in the form of a Real-Time Threat Identification Network ("RTIN") database, which can provide the reputation of source IP addresses, which can be used by customers for filtering network traffic. The source reputation system provides for multiple avenues of access to the source reputation information. Examples of such avenues can include Domain Name Server (DNS)-type queries, servicing routers with router-table data, or other avenues.