Custom email templates for WordPress Plugin

custom-email-template.php

<?php
/**
 * Custom Email Template
 *
 * @wordpress-plugin
 * Plugin Name:       Custom Email Template
 * Plugin URI:        http://wordpress.org/plugins/email-templates
 * Description:       Custom template for Email templates plugin
 * Version:           1.0.0
 * Author:            Damian Logghe
 * Author URI:        https://timersys.com
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 */
 
 // If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

define( 'CUSTOM_MAILTPL_PLUGIN_FILE'   , __FILE__);
define( 'CUSTOM_MAILTPL_PLUGIN_DIR'    , plugin_dir_path(__FILE__) );
define( 'CUSTOM_MAILTPL_PLUGIN_URL'    , plugin_dir_url(__FILE__) );

/**
 * Replace original template with our custom one
 */
add_filter('mailtpl/customizer_template', 'custom_mailtpl_template');
function custom_mailtpl_template( $template ) {
	return CUSTOM_MAILTPL_PLUGIN_DIR . "/templates/default.php";
}