A flexible, event driven and conditional rule based mail messaging system which can be transparently implemented for use in electronic mail applications. A rule mechanism is implemented having a "When-If-Then" event-driven, conditional, action-invoking paradigm or "triplet" which permits definition of a repertoire of events considered to be significant events upon which to trigger actions in the electronic mail messaging system. Each particular event may be associated with a specific mail message and/or rules to promote efficient mapping of messages, events and rules so that only rules associated with a specific event are invoked upon occurrence of the event. Only relevant rules, i.e. those associated with a satisfied event, need be further processed. A graphical user interface to a structured rule editor facilitates synthesis of rules by a user via a substantially transparent rule engine. A modular architecture for the structured rule editors effects an extensible and portable facility invoking selected rule scripting language to implement various functions in the context of various electronic mail messaging environments.