howto-add-custom-woocomerce-email.php

<?php

add_filter( 'woocommerce_email_classes', 'mk_woocomerce_my_custom_email' );

function mk_woocomerce_my_custom_email( $email_classes ) {
	// add the email class to the list of email classes that WooCommerce loads
	$email_classes['Mk_Custom_Mail_Class'] = new Mk_Custom_Mail_Class();

	return $email_classes;
}