woocommerce_mail_callback │ filter-hook │ WC 1.0

Usage

add_filter( 'woocommerce_mail_callback', 'wp_kama_woocommerce_mail_callback_filter', 10, 2 );

/**
 * Function for `woocommerce_mail_callback` filter-hook.
 * 
 * @param  $string 
 * @param  $that   
 *
 * @return 
 */
function wp_kama_woocommerce_mail_callback_filter( $string, $that ){

	// filter...
	return $string;
}

$string

-

$that

-

Where the hook is called

$mail_callback        = apply_filters( 'woocommerce_mail_callback', 'wp_mail', $this );

Where the hook is used in WooCommerce