# 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