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Enterprise Mail Queue can be driven direct from JIRA events as they happen . Events can also drive EMQ and/or via it API from an external source.
JIRA Event Source
Using JIRA as an event source will process Issue Events through the EMQ. When an Issue Event occurs EMQ searches through the active mail queues (the default is always checked last) to find notification schemes that match. If there is a matching mail queue then the Issue Event is processed by this queue.
API Event Source
EMQ can also be driven via its API, through externally selected queues . Enterprise from another plugin. One plugin that uses EMQs API is the Enterprise Message Handler for JIRA. This provides customization of Templates for JIRA users and Email only recipients, those notifications can be digested through EMQ too, by selecting EMQ within JEMH Event Listener configuration (TODO).