exception_class

# File lib/net/smtp.rb, line 995
      def exception_class
        case @status
        when /\A4/  then SMTPServerBusy
        when /\A50/ then SMTPSyntaxError
        when /\A53/ then SMTPAuthenticationError
        when /\A5/  then SMTPFatalError
        else             SMTPUnknownError
        end
      end