ked against this list. If this information matches any entry in the list, the call will simply be dropped. The mentioned list can be built through community efforts. This means that the SIP users who are aware of the open relay servers menace can coordinate their efforts in order to build a list with these proxies that could be used by the SIP providers for blocking any SIP requests going through these servers. If we observe the e-mail case, such community efforts are already available for use [19]. In the above discussion, we provided both advantages and disadvantages of such modules. As a consequence, the corresponding implementation has to be optimized in order to avoid that such modules harm more than help in mitigating SPIT activities. 23.4.1.3 White/Black Lists Module A SPIT black list is a list of SIP URIs and IP addresses of known SPIT users and servers. In practice, black lists are used to block all SIP requests coming from certain SIP sources identified as being used to send spam. White lists are the opposite of black lists. They involve trusted SIP URIs and IP addresses that are always allowed to establish calls and send instant messages, no matter what the content is. White lists allow calls and instant messages coming from a trusted provider to come through, however they do not provide a solution for blocking SPIT. These lists require constant main- tenance to be very effective. If they are not properly maintained, the risk of losing legitimate calls and instant messages is high. Although their simplicity, the white/black lists can be very efficient if they are implemented correctly. In this section, we will discuss how the white/black listing technique can be handled in order to be efficient. 23.4.1.3.1 Building white/black lists To build white and black lists, different techniques (which could be combined) are possible: Using an interface: this interface will allow the user to manage his lists and update them. A web interface could be the best choice. C 2009 by Taylor & Francis Group, LLC A Concept