If there's an nxdomain for mx sendmail etc will fall back to the A - not to my knowledge if there's just a servfail
No idea about exchange, you should be the expert on that, Mike --srs > On 28-Feb-2016, at 9:39 AM, Michael Wise <michael.w...@microsoft.com> wrote: > > And Exchange. > > Aloha, > Michael. > -- > Sent from my Windows Phone > From: Franck Martin via mailop > Sent: 2/27/2016 7:39 PM > To: Mark Jeftovic > Cc: Suresh Ramasubramanian; mailop > Subject: Re: [mailop] proper NULL MX behaviour for MTAs? > > > >> On Sat, Feb 27, 2016 at 7:23 PM, Mark Jeftovic <mar...@easydns.com> wrote: >> >> On 2016-02-27 9:59 PM, Suresh Ramasubramanian wrote: >> > A domain with a null mx may well originate email but will absolutely not >> > receive email - so mail gets trashed at your end as well without staying >> > endlessly on your mall queues >> > >> > You can possibly correlate that mx with the behavior of domains that are >> > sending you mail.. Though a domain rather than IP bl may make sense. >> > >> >> Sorry, I realized after I sent that I needed to clarify that what I'm >> observing are MTAs attempting to deliver email to addresses at IP of the >> A record for the domain, ignoring the presence of its NULL MX. >> >> So the originating MTAs are ignoring the NULL MX and attempting to >> deliver to the A hostname, leading me to surmise they are spambots or >> zombies. > > This could be true but then RFC7505 is recent, so I'm not sure that proper > MTAs are yet properly behaving. I wonder what sendmail, postfix, exim really > do?
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