Question : 451 4.4.0 DNS query failed error in Exchange 2007 queue viewer

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.

Answer : 451 4.4.0 DNS query failed error in Exchange 2007 queue viewer

mx record in internal DNS pointing to server02.internaldomain.com > Exchange Server ?
Can you delete that record.

Go here
www.mxtoolbox.com
Enter your domain name
Check the first MX record > MAIL.domain.com
This should correspond to your public IP -

mx record external should point to
mail.domain.com > points to your IP address > Goes to Firewall / Router > Port Forwards to server02.internaldomain.com

Also ports 25 80 443 are open
Can be verified by going here

www.canyouseeme.org

thanks