This weekend we updated our DNS to a different ISP. We made the change Friday night around 6 pm and it updated Saturday night around 6 pm. Emails are coming through when it is slow, but during the peak hours it slows down. Even internal emails are taking 1-2 hours to arrive. I do not know what to look for to see what is causing the problems. There are about 4000 emails in the Queue Viewer on the Exchange Management Console.

Just for your information... we use a fatpipe system to bond three different ISP lines. Traffic is aggregated depending on which line has the best latency.

When we were making the change we removed the DNS forwarders on the exchange server that went to our old line, and added the DNS info for the new line. It seems to me that we would need all 3 DNS information on the exchange server, does that seem right?

Does anyone know what I should try to determine what is causing the queuing? Any help is greatly appreciated.

  • I'm at a loss to determine why an external DNS change would affect internal emails. Can you give us more info regarding the internal DNS configuration and more specifically the DNS configuration on the Exchange server? Also, what do you mean you changed the DNS forwarders on the Exchange server? Exchange doesn't use DNS forwarders. – joeqwerty Mar 15 '11 at 1:26
  • Go to DNS Manager, right click on the server, select properties, forwarders. – Mr. Monkey Mar 15 '11 at 14:42
  • Exchange server doesn't have a DNS manager. Is the Exchange server also your DC\DNS server? – joeqwerty Mar 15 '11 at 19:33

Viewing an individual message in the Queue Viewer should show you which error code was returned when delivery was attempted.

For DNS, since your traffic may come out ANY pipe, and oftentimes ISPs will only allow queries to their recursive resolvers that originate from their OWN pipe, this could be a likely culprit for why message delivery is intermittent (successful when the new ISP's pipe is used, failure when one of the others is used).

I'd suggest changing over to an open, global DNS resolver; Google runs some that are solid, for instance.

  • This sounds a likely cause, I would change this and see if it cleans itself up. – Ethos Mar 14 '11 at 22:15
  • On most of the messages I am getting a lot of 400 4.4.7 and 451 4.4.0 errors. And it sounds like those are mostly DNS errors. The only confusing thing is it shows all of our outbound emails are SPAM emails. I've heard of viruses that will just send mass emails out like that. What do you think? – Mr. Monkey Mar 14 '11 at 22:26
  • 451 4.4.0 seems to be DNS query failed, so I would definitely try changing to public NS like google ( Regarding the SPAM, do the messages look legit? How do the internal clients send mail and how is relaying setup on the SMTP virtual server? – Ethos Mar 14 '11 at 22:48
  • I noticed yesterday a lot of the errors for reverse lookup. I called our ISP and had them set up the reverse DNS. So that should help with most of the DNS errors. I can add a google DNS no problem. **EDIT: Where do I add a DNS? Right click on server - properties - forwarders? Also, it looks like my submissions queue on the server is full. It has 1200 emails from yesterday that still cannot process. 99% are spam, 1% are legit. It will not let me resume or delete the emails. What gives? – Mr. Monkey Mar 15 '11 at 14:46

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