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.