RESOLVED. In our case, it turned out our ISP had implemented some kind of DNS grey listing policy, in which DNS packets between [client IP, server IP] pairs were blocked unless there was a preceding attempt in the last 60 seconds. (see http://en.wiki...

There was no non-authoritative reply.  MxToolbox displays "NON-AUTH" even though really the query timed out.

So I'm on the other end - seems anyone with an Ironport has intermittent MX query failures to our domain Hitchcock.org.We've had a few critical partners set up SMTP routes to our mail servers, or as noted here have the Ironport query their internal D...