1. A network traffic filtering system for filtering a flow of electronic messages across a computer network, the system comprising: one or more computing devices; an engine associated with a centralized server installed on the one or more computing devices, the engine executing instructions stored in a memory of the one or more computing devices, the execution of the instructions configured to: generate reputation data and new reputation data for sources of messages by evaluating messages after being sent from a sending server and before being received by a targeted receiving server; generate a source reputation profile for a source of a message by evaluating reputation data associated with that source of that message, wherein this evaluation comprises generating a reputation score for the source, the reputation score indicating a likelihood that electronic messages from the source are wanted or unwanted, and update the source reputation profile by evaluating new reputation data associated with the source, wherein this evaluation comprises generating an updated reputation score for the source, and wherein an updated reputation score different than the reputation score indicates a change in the likelihood that electronic messages from the source are unwanted; a profile database associated with the engine for storing the reputation data, the reputation score, the updated reputation score, and the source reputation profile; and the centralized server connected to the profile database and external to the receiving server and a terminal connected thereto for receiving incoming messages from the receiving server, and the centralized server configured to provide the source reputation profile to an external local system including the receiving mail server, the source reputation profile for filtering incoming electronic messages received by the receiving mail server.