Specifies how to rewrite the source address. Digits will be set directly to the new address. A '.' will move a digit from the old address to the new address. A '*' will move all remaining digits from the old address to the new address. A '%' will delete a digit from the old address. A '#' will append all digits from the old address to the new address. Please note that the eventually leading '@' (indicating extended adressing) is handled execatly like digits. If this object is empty, the source address will not be changed.