If DIR/headerkeep (*5.1) is present, all headers other than what are listed are removed. Otherwise, all headers listed in DIR/headerremove are removed from all messages before they are sent to the subscribers. Some of these must be removed in order for the list to function. You can add others (see More Information). If you add a header in DIR/headeradd that should not occur multiple times, it is good to add the same header to DIR/headerremove to have it removed from incoming messages, if it exists.