On 7 Nov 2018, at 15:27, James B. Byrne wrote:
Neither dns02 nor dns04 are listed in the /etc/resolv.conf file on the affected services.
That does not necessarily mean they are not being tried. They are half of your authoritative nameservers and they aren't working, so unless the nameserver(s) in resolv.conf are authoritative for harte-lyne.ca or you have a split-horizon DNS setup, sometimes you'll ask the missing one or the broken one or (worst) the broken one and then the missing one. If your resolver retry and timeout settings are strict, it may give up before getting any answer other than SERVFAIL.
With respect to Viktor's answer. My understanding is that: in the absence of a specified MX record then the A RR is supposed to be used. In this case MX31 is one of the MX for the entire domain. Why is the failure to lookup an MX record fatal? Why is not the A record value used in its absence?
A timeout or a SERVFAIL for the MX lookup is not an authoritative result. The MX record may exist, but not be accessible. If MX lookup fails non-authoritatively, fallback to the A record is not correct.
-- Bill Cole
