Null mx would be a plug and play logical signal, condition to stop, preempt, do not queue, skip the outbound "send mail" process for the email domain because this proposal says its an indicator there would be no listening server ip available. In other words, it's an optimizer. It would be networking waste to do make the call. The issue I have are false positives. Do we know for a fact, there are no legitimate mx records with explicit preference zero settings with legitimate ip receivers? Should we believe them? I might suggest a better algorithm where a first time attempt is made to double check if there is no legitimate ip or server to connect to. A implicit mx attempt is already done to block the domain by our software and tony also mentioned the same in Exim. I assume it is a long time common thing. I think I can say for a support fact that there are false positives, but maybe not significant to not kill the idea of a single shot attempt. I think the "dynamic DNS domain" services are some of the main reasons for intermittent errors. The few times over the years were handled manually when reported. The pref equal zero offers a new rule to avoid the single shot attempt. But I am not sure if it's worth the coding change the avoid the outbound mail attempt. Note, I know in our software we reduced the TCP/IP timeout from a default if about 30 secs to 10 secs depending on the socket stack so it's less of a loading/performance concerns. -- Hector Santos http://www.santronics.com > On Jul 17, 2014, at 9:06 PM, Dave Crocker <dhc@xxxxxxxxxxxx> wrote: > >> On 7/17/2014 6:01 PM, Viktor Dukhovni wrote: >> In find nullmx on the contrary far less objectionable than all >> other anti-abuse measures. > > Although it has some utility for anti-abuse, it is not an anti-abuse > mechanism. It is of more general benefit. > > >> I just designates non-sending domains, > > No, it designates non-/receiving/ domains: > > Abstract: > > "The NULL MX RR formalizes the existing > mechanism by which a domain announces that it accepts no mail," > > d/ > > -- > -- > Dave Crocker > Brandenburg InternetWorking > bbiw.net > >