A system and method for controlling the use of addresses by using address computation techniques is described. A system comprising alias address creation software generates multiple alias addresses representing a single real address of a particular recipient. Each alias address is computed from data representing a prospective sender and a recipient. A sender is provided with an alias address by a recipient for communicating back to said recipient. Messages sent by a sender, employing alias addresses are analysed to a forwarding server which validates each alias address and checks it against a blocking list. Messages which pass these checks are directed to the recipient's real address registered with said forwarding server.