A system that enables a communications system user to conveniently define entities to be excluded from receiving an electronic communication, such as an electronic mail (“email”) message. An “Exclude” user entry field is provided in a user interface to a communication system. The Exclude field accepts entry of an “exclude set” of entities to be excluded from an email message being processed or defined in a currently accessible graphical user interface window. The disclosed Exclude field is provided in a user interface window together with one or more other user entry fields available for definition and/or review of an “include set” of entities initially designated for receipt of the message. Lists and/or groups may be resolved by determining the destinations they contain either within the client computer system, within a source server computer system, and/or within a destination server computer system.