"451 4.4.0 DNS query failed" error in Edge Transport

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  We are using Exchange 2007 SP1. The topology is:
  
  - 2 x mailbox servers running in CCR at internal network.
  - 1 x Hub Transport + CAS at internal network
  - 1 x Edge Transport at DMZ
  
  Yesterday, we found that emails sending to two domains (say a.com and b.com) were queued up in the Edge Transport with error "451 4.4.0 DNS query failed". I checked the "send connector" and it uses "External DNS" to resolve domain names. I then checked the "External DNS" definition in both Edge and Hub Transport and found that the DNS entries were set correctly (which is pointing to our ISP's DNSs).
  
  With our AD, we have DNS, too. However, it is for internal use only. Within this internal DNS, we have both a.com and b.com zone records defined but there is no MX record in them.
  
  Strangely, after removing the zone records in our internal DNS, the pending emails can send out. After all the pending emails were gone, we rebooted the Edge Transport and add back the zone records of a.com and b.com to our internal DNS. After that, emails sent to these two domains could go through without problem.
  
  This is really a strange case. Does anybody come across this? Any reason for this issue?
  
  Joseph

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  Joseph Liu

  2009年5月8日 上午 02:40