# File lib/mail/message.rb, line 1392
    def add_content_transfer_encoding
      if body.only_us_ascii?
        header[:content_transfer_encoding] = '7bit'
      else
        warning = "Non US-ASCII detected and no content-transfer-encoding defined.\nDefaulting to 8bit, set your own if this is incorrect.\n"
        STDERR.puts(warning)
        header[:content_transfer_encoding] = '8bit'
      end
    end