check_response

# File lib/net/smtp.rb, line 670
    def check_response( res, allow_continue = false )
      return res if /\A2/ === res
      return res if allow_continue and /\A3/ === res
      err = case res
            when /\A4/  then SMTPServerBusy
            when /\A50/ then SMTPSyntaxError
            when /\A55/ then SMTPFatalError
            else SMTPUnknownError
            end
      raise err, res
    end