Simple Email Templates for Jira
Simple Email Templates provides users with option to send custom notification emails on issue workflow transitions, using custom templates.
This can be handy when you do not want to use existing JIRA templates or wish to send emails to external persons, that are not registered as users in your JIRA.
Download and install from Atlassian Marketplace.
Plugin consists of a basic template manager/editor and post function where you can configure what template to use and whom to send the email.
Post Function Configuration
For options on what values from issue can be used in the email template refer to Email Template Context.
Refer to Email Template Example.