# File lib/mail/message.rb, line 1356
    def method_missing(name, *args, &block)
      #:nodoc:
      # Only take the structured fields, as we could take _anything_ really
      # as it could become an optional field... "but therin lies the dark side"
      field_name = underscoreize(name).chomp("=")
      if Mail::Field::KNOWN_FIELDS.include?(field_name)
        if args.empty?
          header[field_name]
        else
          header[field_name] = args.first
        end
      else
        super # otherwise pass it on
      end
      #:startdoc:
    end