    # File lib/action_mailer/vendor/tmail-1.2.3/tmail/mailbox.rb, line 391
        def initialize( dir = nil )
          @dirname = dir || ENV['MAILDIR']
          raise ArgumentError, "not directory: #{@dirname}"\
                                  unless FileTest.directory? @dirname
          @new = "#{@dirname}/new"
          @tmp = "#{@dirname}/tmp"
          @cur = "#{@dirname}/cur"
        end