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451 4.4.0 DNS query failed error in Exchange 2007 queue viewer

kobymaster asked
Last Modified: 2013-11-30
We have a 2003 AD and DNS Server01 and an Exchange 2007 Server02. Whenever someone in the domain sends email to an external recipient it end us in the queue with the 451 4.4.0 DNS query failed error. It sits in there for about 10-20 minutes and then goes out. Some emails sit in there for hours and some go out immediately (e.g.e to @yahoo and @gmail recipients). Is there anything I can do to speed up the process? Why is Exchange temporarily holding some emails in its queue? Do I really have a DNS issue and what can I do to fix this? Internal email is flowing well and inbound email is flowing without any problems. Thanks.
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