# File src/rails-1.1.5/actionmailer/lib/action_mailer/vendor/tmail/mail.rb, line 320
    def add_hf( name, field )
      key = name.downcase
      field = new_hf(name, field)

      if ALLOW_MULTIPLE.include? key
        (@header[key] ||= []).push field
      else
        @header[key] = field
      end
    end