A Voice over IP (VoIP) or Real Time Messaging (RTM) firewall device is claimed that protects VoIP or RTM network traffic by identifying and controlling the delivery of such network traffic that is unsolicited and undesired by the recipient (i.e. VoIP or RTM spam). The system involves applying a unique marking to RTM messages close to a point of message origination and then at a point close to message termination for the intended recipient examining a reputation store for information on the unique marking and using that information in conjunction with a set of policy rules to decide whether to pass, reject, pass on to an RTM store or otherwise filter the RTM message. The unique marking serves to identify a source characteristic of the message such as the message originator, a corporate affiliation for the originator, or a RTM network characteristic of the originator such as a transmission gateway.