You can change default email template used by the plugin from the settings. You can use variables from reminder itself or issue to add information to email body or subject. List of available variables are also available inside plugins template configuration page. Click on the question mark icon next to template to display it. Templates are Velocity templates, you can use any Velocity keyword inside the template. If you want to return to default template, just delete everything and save as empty, it will automatically revert to default template.
By clicking on the little question mark icon next to template field, you can display variables you can use in the templates and their description.
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