Exchange 2007 Difficulties delivering email to some domains – Error 451 4.4.0 DNS query failed

Here is an interesting one but quite repeatable…

I’ve run into this situation now in two totally different Exchange 2007 environments (the source). While we are able to successfully deliver messages to most customer domains, in these two instances, one was a domain in BC and the other was in Argentina.

The Problem

For whatever reason, Exchange (and DNS) is hyper sensitive to a DNS domain that provides a bogus response (say one Name Server is healthy, the other is not). It appears to be an issue where we need to wait until the DNS TTL expires or, we perform the ‘solution’ below. This is a temporary measure as we work with the end user domain, to find out what is going on with their DNS (in both cases, after flushing DNS locally and my AD DNS server, I would get a different DNS NS result, and sometimes an MX record, sometimes not … strange, but their problem). However, Exchange 2007 seems to be especially sensitive to this…

For example, with the domain below, here is the error I’ve been experiencing (in the Exchange 2007 Queue…) – 451 4.4.0 DNS query failed

The Solution

Or should I say, the workaround. The solution is contact the destination DNS/domain provider and have them fix their problems (though perhaps a future fix from Microsoft/Exchange will resolve this, or maybe they have and I’m somehow unable to stumble across the answer). However, the workaround is…

  1. Flush your Active Directory DNS Cache (to do this, you’ll need to have ‘advanced’ turned on, the View option in DNS)

  1. On your Exchange 2007 Server(s), flush your local DNS Cache


For me, repeatedly, when I click ‘retry’ in the Exchange 2007 Message queue, the message leaves immediately. Sure enough, a few hours later I have a crap record but it does at allow the messages to flow. Now to get the target domain/DNS provider to cooperate and recognize they have a problem, of some sort…

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